The United States has made a preliminary conclusion that Russia knew in advance of Syria's chemical weapons attack last week, but has no proof of Moscow's involvement. That's according to a senior U.S. official.

The official said Monday that a drone operated by Russians was flying over a hospital as victims of the attack were rushing to get treatment. Hours after the drone left, a Russian-made fighter jet bombed the hospital in what American officials believe was an attempt to cover up the usage of chemical weapons.

The official said the presence of the surveillance drone over the hospital couldn't have been a coincidence.