Russia struck the Islamic State stronghold of Raqa in Syria on Tuesday with a "significant number" of strikes that included long-range bombers and sea-launched cruise missiles, US defense officials said.
"We are aware that over the past several hours Russia conducted a significant number of strikes in Raqa, some of which may have included sea-launched cruise missiles and long-range bombers," said one official, who made the remarks on condition of anonymity.
Russia's Defense Minister Sergei Shoigu said that powerful long-range bombers had been used to strike targets around the jihadist strongholds of Raqa and Deir Ezzor, and to fire cruise missiles at Idlib and Aleppo regions.
A second US official, again speaking on condition of anonymity, later told AFP that Russia had sent one "strike package" of long-range bombers and had also fired "about 20" cruise missiles from the Caspian Sea, all of which appeared to have landed in Syria.
He said the Russian bombers flew to Syria across Iraqi air space.
Russian President Vladimir Putin earlier vowed vengeance as Moscow confirmed a bomb brought down a Russian passenger jet over Egypt last month, killing all 224 people aboard.
When Russia fired cruise missiles at Syria last month, the Pentagon said four of them had crashed in Iran. Moscow denied the claim.
The United States and Russia have been waging separate air campaigns in civil war-torn Syria, and US officials said Moscow gave it prior warning before carrying out Tuesday's strikes.
In the days after Russia's bombing campaign began, Pentagon and Moscow officials drew up flight safety guidelines specifying how pilots should act if they cross paths.
In a meeting with his security chiefs, Putin pledged to ramp up Moscow's bombing campaign in Syria in response to the attack on the passenger plane. But he but stopped short of blaming any one group.
"We will search for them anywhere they might hide. We will find them in any part of the world and punish them," he said.