Russia 'knew in advance' of Syrian gas attack

Russian military chiefs had advance warning that Syria was planning a chemical weapons attack on its citizens but did nothing to stop it, the US has concluded.

The explosive claim was made by a senior American official who told media agency AP the Pentagon had detailed reports of a Russian operated drone flying over the site of the deadly incident in the northern Syrian town of Khan Shaykhun.

The drone flew up straight after the April 4 suspected sarin gas attack, and hovered over a local hospital as victims poured in, reports The Daily Telegraph in the UK.

Five hours after the mass gassing, the camera switched off and a Russian-made aircraft struck the hospital. US intelligence officials believe that air strike may have been an effort to cover up the initial gas attack.

The official, who insisted on anonymity, said it was not clear who was flying the jet that bombed the hospital. Both the Syrian and Russian air forces fly the same Russian-built planes.

But the official said the presence of the drone could not have been a coincidence,  and that Russia must have known the chemical weapons attack was about to take place. The official believed the drone was flying near the town to help with the aftermath.

The chemical attack left at least 80 people dead and caused worldwide outrage.

After seeing horrific images of children gasping for breath, Trump reacted with force.

He ordered a volley of Tomahawk missiles at the Syrian air base where the chemical attack was hatched.

The latest claim of potential Russia involvement dramatically raises the tension for the meeting between US Secretary of State Rex Tillerson and his Kremlin counterpart Sergei Lavrov.

They meet in Moscow today.