Russia knew in advance that Syria was planning a chemical weapons attack on its own people but did nothing to prevent it, the US has concluded.

A senior US official told AP that Washington had analysed reports of a drone flying over the site, and determined that the drone was operated by the Russians.

The drone went up immediately after the April 4 sarin gas attack, and begun hovering over a local hospital as victims began pouring in. Five hours after the sarin attack, the camera switched off and a Russian-made aircraft struck the hospital. The two military officials said that it was unclear whether the aircraft were Russian or Syrian as the Syrians buy Russian-made aircraft.

US officials believe the jet bombed the hospital in an attempt to cover up the usage of chemical weapons.