A homemade explosive device brought down a Russian passenger plane over Egypt last month, the head of Russia's FSB security service said Tuesday, telling President Vladimir Putin it's now clear the bombing that killed 224 people was a "terrorist" act.

Putin vowed to hunt down those responsible for the attack, which the Islamic State group claimed to have carried out. Two U.S. defence officials said Russia struck Raqqa, the Islamic State group's self-declared capital in Syria, with cruise missiles and bombs on Tuesday.

Russian authorities allege bomb brought down passenger plane 1:19

The FSB offered a $50-million reward for information leading to the arrest of those responsible, appealing to the "Russian and international communities for co-operation in identifying the terrorists." The FSB specified that the reward would be paid in dollars.

"According to our experts, a homemade explosive device equivalent to one kilogram of TNT went off onboard, which caused the plane to break up 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," FSB head Alexander Bortnikov said.

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

All of the people on board, most of them Russian tourists, were killed when the Metrojet Airbus 321-200 crashed over the Sinai Peninsula on Oct. 31, about 23 minutes after taking off from the Red Sea resort city of Sharm el-Sheikh. The plane was headed to St. Petersburg, where most of the passengers were from.

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.

Reuters reported Tuesday that Egypt has detained two employees of Sharm el-Sheikh airport in connection with the Metrojet crash, citing two unnamed security officials and an airport employee. However, the country's interior and civil aviation ministries both denied that any arrests had been made.

Retaliation for airstrikes

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. defence officials said the Russians gave prior notification of their attacks on Raqqa by communicating with the U.S.-led coalition's air operations centre 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.

Russian President Vladimir Putin chairs a Tuesday meeting at the Kremlin about Metrojet plane crash in Egypt. The Kremlin said for the first time that a bomb ripped apart the passenger jet over Egypt last month. (Alexei Nikolskyi/SPUTNIK/Kremlin/Reuter)

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

Not considering ground troops

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-defence. Everyone who tries to aid the criminals should understand that they will be responsible for giving them shelter."

Putin was unusually sombre. 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.​