Thousands of Syrians are gathering at the border with Turkey after fleeing airstrikes in Aleppo.
The city has been split between rebel and government control over the last few years - with intense battles in recent weeks.
Russia is carrying out airstrikes in support of the Syrian army.
US Secretary of State John Kerry is accusing Moscow of indiscriminate bombing: “There are civilians including women and children being killed in large numbers.
“In some cases after the bombing has taken place when the workers have gone in to pull out the wounded.
“The bombers come back and kill the people that are pulling out the wounded, this has to stop.”