“We can say definitely that this was a terrorist act,” Alexander V. Bortnikov, the head of the Federal Security Service, or F.S.B., said in remarks to Russia’s Security Council.
An “improvised explosive device” had detonated soon after the plane took off from the resort city of Sharm el Sheikh, he said in remarks broadcast nationally. “The plane disintegrated in midair, which explains the widely scattered fuselage pieces.”
British and American inteligence agencies strongly suggested that a bomb had brought down the Metrojet Airbus A321 just days after it went down on Oct. 31. The Islamic State, also known as ISIS or ISIL, claimed responsibility for the attack within hours, although it did not provide any proof.
The announcement from the Kremlin was the first, clearly definitive statement from Russia that the plane was brought down by a terrorist act, although leaders had been moving in that direction after initially criticizing the early suggestions a that a bomb was responsible.
Asked if Russia had concluded that the Islamic State was behind the attack, Dmitri S. Peskov, the Kremlin spokesman, said the group had exported terrorism to other countries but that Russia could not say definitively that the Islamic State was responsible, the RIA Novosti news agency reported.
Most tourist flights to the Red Sea resort have been suspended since the crash because of security concerns that a member of airport staff was able to slip the bomb on board.
The bomb contained up to 1 kilogram, or 2.2 pounds, of TNT, Mr. Bortnikov said, adding that “foreign made” explosive material was found on parts of the plane and other objects that were examined.
Russia has offered $50 million for any information leading to the capture of those who carried out the attack, and President Vladimir V. Putin vowed to track them down.
“We will search for them everywhere, no matter where they are hiding,” Mr. Putin said at a meeting with his security council that was broadcast on national television. “We will find them in any place on the planet and will punish them.”
Mr. Putin said the attacks by the Russian air force in Syria would not only continue but intensify.
“Our military work in Syria must not only be continued, but strengthened so that criminals understand that punishment is inevitable,” Mr. Putin said.
The Ministry of Defense and the military had been ordered to draw up plans, he said.
In claiming responsibility, the Islamic State branch on the Sinai Peninsula said the attack came in retaliation for Russia’s deployment of its military in Syria, where it is trying to shore up the rule of President Bashar al-Assad by attacking his opponents.
A spokesman for Egypt’s president, Abdel Fattah el-Sisi, did not immediately respond to messages seeking comment on the Russian statement.
Egyptian officials have dismissed the possibility of terrorism, sometimes suggesting that theories of a militant attack are part of an international conspiracy against their country.
Confirmation that a bomb brought down the plane — and was presumably smuggled through the Sharm el-Sheikh airport — would represent a devastating blow to the country’s tourism industry, and would undermine government claims that the authorities are prevailing in the war against militants based in the Sinai Peninsula.