MOSCOW (AP) — A somber President Vladimir Putin vowed to hunt down and punish those responsible for a bomb that brought down a Russian passenger jet last month, "wherever they are hiding." Intensified Russian airstrikes Tuesday hit the Islamic State group's stronghold in Syria that also is being pounded by the French military.

Russia's FSB security service confirmed for the first time that a homemade bomb caused the Oct. 31 crash of the Metrojet Airbus 321-200 in Egypt's Sinai Desert and killed all 224 people aboard in what it called "a terrorist act." The Islamic state had already claimed responsibility, saying it was punishing Moscow for its air campaign in Syria.

With Russia striking IS targets in reprisal for the Metrojet downing, France hitting militants in response to the attacks on civilians in Paris last week and the ongoing U.S.-led coalition bombings, the extremists were facing a stepped-up assault from at least three military powers. But the Kremlin said it was too soon to call it a de facto alliance.

Putin heard the report on the Metrojet bombing at a late-night meeting of his security advisers in the Kremlin that was shown on national television. He and his advisers stood for moment of silence for those who died on the plane, mostly Russian vacationers flying from the Red Sea resort of Sharm el-Sheikh to St. Petersburg.

An unusually subdued Putin called it "one of the bloodiest crimes," and promised retribution for "the murder of our people in Sinai."

"We will not wipe away the tears from our souls and our hearts. This will stay with us forever," he said.

The FSB offered a $50 million reward for information leading to the arrest of those responsible, and Putin said Russia would be relentless in its pursuit.

"We're going to look for them everywhere wherever they are hiding. We will find them in any place on Earth and punish them," he said.

FSB head Alexander Bortnikov said a bomb containing the equivalent of 1 kilogram (2.2 pounds) of TNT went off aboard the Airbus, causing it to break apart in the air, which explains why the fuselage was scattered over such a large territory. I can certainly say that this was "a terrorist act," Bortnikov said.

He said tests showed the explosives had been produced outside of Russia, but gave no other details.

Egyptian Prime Minister Sherif Ismail said his government was informed only on Tuesday of the results of the Russian investigation, and he shied away from endorsing its conclusion that a bomb was to blame. Speaking in Sharm el-Sheikh following a Cabinet meeting there, Ismail said Egypt would "bear in mind" the Russian conclusion.

Aviation Minister Hossam Kamal, meanwhile, sought to cast doubt on the Russian declaration, saying the technical investigation into the crash by experts from Egypt, Russia, France and Ireland had not concluded.

Egyptian authorities opened an investigation into a possible security breach or infiltration by militants of the Sharm el-Sheikh airport staff, security officials said. The investigation has focused on baggage handlers, their security supervisors and also personnel involved in aircraft catering, according to the officials, who spoke on condition of anonymity because of the sensitivity of the case.

Egypt is concerned about the impact on its tourism industry. Sharm el-Sheikh was particularly popular with Russian tourists, who in 2014 accounted for nearly a third of the nearly 10 million visitors to Egypt.

Russia has suspended all flights to Egypt. Russia also has banned Egypt's national carrier from flying to Russia.

Seeking to reassure travelers, Egyptian Interior Minister Magdy Abdel-Ghaffar said security measures at airports had been stepped up nationwide.

Putin's meeting at the Kremlin was held immediately upon his return from Monday's summit with President Barack Obama and other world leaders in Turkey, where they vowed to work together to combat the Islamic State group following Friday's attacks in Paris that killed 129 people. IS has claimed responsibility for those bombings and shootings.

Putin ordered the Russian missile cruiser Moskva, currently in the Mediterranean, to start cooperating with the French military "as allies" on operations in Syria. He said a French aircraft carrier task force was to approach the Moskva soon.

The plane crash and the Paris attacks clearly have raised Russia's determination to fight IS, although Western concerns remain that its airstrikes also are hitting rebels who oppose Syrian President Bashar Assad but are not affiliated with radical groups.

President Barack Obama said the U.S. would "very much want to see" Russia shift its military focus in Syria to targeting the Islamic State and will be "in discussions with Moscow, Mr. Putin, to see if that will continue."

"We're going to wait and see whether in fact Russia does end up devoting more attention to targets that are ISIL targets and if it does so that's something we welcome," Obama said Wednesday while traveling in the Philippines.

Putin said that Russia's air campaign in Syria "should not only be continued but should be intensified so that the criminals realize that retribution is inevitable."

Defense Minister Sergei Shoigu said Russian bombers hit positions in the Islamic State's stronghold of Raqqa, as well as in Deir al-Zour.

Russian warplanes fired cruise missiles on militant positions in Syria's Idlib and Aleppo provinces, Shoigu said. The Islamic State group has positions in Aleppo province, while the al-Qaida-linked Nusra Front is in Idlib province.

The cruise missiles that hit the Aleppo and Idlib positions were fired from supersonic Tu-160 bombers and from Tu-95s, long-distance turboprop strategic bombers that started from Russian territory, he added.

France's airstrikes Tuesday in Raqqa destroyed a command post and training camp, said French military spokesman Col. Gilles Jaron. Later, Defense Minister Jean-Yves Le Drian said there were new raids in the evening by 10 fighter jets. The aircraft carrier Charles de Gaulle will depart Toulon on Thursday for the region.

French President Francois Hollande, who stayed away from the G-20 summit because of the Paris attacks, said he would travel to Moscow next week to help forge a military coalition against the Islamic State group.

France also made an unprecedented demand that its European Union allies support its military action against IS. It invoked a never-used article of the EU's Lisbon Treaty obliging members of the 28-nation bloc to give "aid and assistance by all the means in their power" to a member country that is "the victim of armed aggression on its territory."

Le Drian said all EU partners responded positively, and they could help "either by taking part in France's operations in Syria or Iraq, or by easing the load or providing support for France in other operations."

Kremlin spokesman Dmitry Peskov was asked by reporters if Putin's order about the Moskva cooperating with the French meant there was a de facto coalition on Syria involving Moscow and Paris. He replied: "For now, it's premature to say this."

Putin said Russia is "relying on all of our friends."

"We will act in accordance with the U.N. Charter's Article 51, which gives each country the right to self-defense. Everyone who tries to aid the criminals should understand that they will be responsible for giving them shelter," he said.

An image posted on the Facebook page of Russian Foreign Ministry spokeswoman Maria Zakharova apparently sought to show solidarity between Russia and France. Similar to the popular meme of the Eiffel Tower inside a circle forming a peace symbol on the French tricolor, the image on Zakharova's page shows a jet plane in a circle superimposed on the white, blue and red of the Russian flag.

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Associated Press writers Nataliya Vasilyeva and Jim Heintz in Moscow, Thomas Adamson in Paris, Kathleen Hennessey in Manila, Philippines, and Hamza Hendawi in Cairo contributed to this story.

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A previous version of this story has been corrected to show that the spelling of the Egyptian city is Sharm el-Sheikh, not Sharm al-Sheikh.

Copyright © The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

In Cairo, there was no immediate comment on the news from the Egyptian government. State-owned television carried the news from Moscow, but had no official comment either.

Egypt had resisted British and U.S. assertions that an explosive device was the likely cause of the Russian plane's crash. Later, government officials and the pro-government media shifted their focus away from the cause of the crash to speculating on what they called a Western conspiracy against Egypt and the crushing impact of the crash on the country's vital tourism industry.

Hunting down those responsible

Putin said Russia would be relentless in hunting down those responsible for the attack.

"There's no statute of limitations for this. We need to know all of their names," Putin said. "We're going to look for them everywhere wherever they are hiding. We will find them in any place on Earth and punish them."

The Islamic State group had claimed responsibility in written statements, as well as video and audio messages posted on the Internet following the crash. It said the attack was retaliation for Russia's air campaign against IS and other groups in Syria, where Moscow wants to preserve the rule of President Bashar Assad.

One of the U.S. defense officials said the Russians gave prior notification of their attacks on Raqqa by communicating with the U.S.-led coalition's air operations center at al-Udeid air base in Qatar. The official said this was in accordance with protocols worked out by U.S. and Russian officials in October.

The officials, who spoke on condition of anonymity to discuss non-U.S. military operations, provided no details on targets or the scale or outcome of the attacks.

Putin said Tuesday that Russia's air campaign in Syria "should not only be continued but should be intensified so that the criminals realize that retribution is inevitable."

He instructed the Defense Ministry and General Staff to present their suggestions on how Russia's operation in Syria could be modified.

Putin spokesman Dmitry Peskov said the president's order didn't mean that Russia was considering sending ground troops to Syria.

Putin's meeting with Bortnikov and other senior officials was held late at night, shortly after the president returned from meetings with other world leaders in Turkey. The leaders from the Group of 20 rich and developing nations had vowed to work together to combat the Islamic State group.

"In this work, including the search to find and punish the criminals, we are relying on all of our friends," Putin said. "We will act in accordance with the U.N. Charter's Article 51, which gives each country the right to self-defense. Everyone who tries to aid the criminals should understand that they will be responsible for giving them shelter."

Putin was unusually somber. After Bortnikov pronounced it a terrorist act, Putin asked the other officials around the table to stand for a moment of silence.

IS has warned Putin that it would also target him "at home," but did not offer any details to back its claim. While releasing specifics would add credibility, the group may be withholding because its claim is false, because doing so would undermine plans for similar attacks in the future, or because the aura of mystery might deepen its mystique among die-hard followers.

IS has also claimed responsibility for Friday attacks in Paris that killed 129 people and wounded 350 others.

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Nataliya Vasilyeva in Moscow, Hamza Hendawi in Cairo and Robert Burns in Washington contributed to this story.

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A previous version of this story has been corrected to show that the spelling of the Egyptian resort city is Sharm el-Sheikh, not Sharm al-Sheikh.

Copyright © The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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