US officials said they were seeing signs that thousands of troops from across the Shia world -  including Syrian regime soldiers, Iranian Revolutionary Guards, Hizbollah fighters, Iraqi militiamen and Afghan mercenaries - were massing for a final assault on Aleppo. 

The warning came as largest hospital on the rebel side of the city was put out of service after being hit by barrel bombs on Saturday amid a dramatically intensified Russian and Syrian regime bombardment of Aleppo.

Boris Johnson, the foreign secretary, accused Russia of carrying out illegal "double tap" strikes, in which an area is bombed and then struck again minutes later to kill first responders as they try to rescue the injured. As US-led diplomatic efforts to broker a truce continued to stall, Moscow warned Washington against intervening militarily against the Assad regime, saying that American strikes against Damascus would have "terrible, tectonic consequences".