Story highlights Strikes have come by sea and by air

Coalition operations in Syria not affected, U.S. official says

(CNN) A U.S. defense official says Russia has conducted a "significant number of strikes" in Raqqa in northern Syria in the past several hours. The terrorist group ISIS, which has claimed responsibility for downing a plane bound for Russia in Sinai, Egypt, last month, has made its Syrian headquarters in Raqqa.

"We are aware that over the past several hours, Russia conducted a significant number of strikes in Raqqa, some of which may have included sea-launched cruise missiles and long-range bombers," the official said.

"While we do not coordinate or collaborate in any way with Russia on its activities in Syria, I can confirm that the Russians did provide us notice prior to conducting these strikes, via the Coalition Combined Air Operations Center in Qatar, in accordance with the safety protocols agreed to in October," the official said.

The notification is meant to help prevent accidents. The official said the Russian strikes are not affecting coalition operations against ISIS in any way.