US President Barack Obama and German Chancellor Angela Merkel condemned what they called "barbarous" Russian and Syrian regime airstrikes on Aleppo during a phone call Thursday, the White House said.

The Syrian government and its ally Russia "bear special responsibility for ending the fighting in Syria," the two leaders agreed, strongly condemning the strikes in eastern Aleppo, an area they said is "populated with hundreds of thousands of civilians, half of whom are children."

The United Nations says a humanitarian catastrophe is unfolding unlike any witnessed so far in Syria's brutal five-year war as Moscow and Damascus carry out an offensive to retake the city after a ceasefire collapsed last week.

Russia and the United States have traded blame for the collapse of the deal that would have marked the first step in a new effort to end the war that has killed 300,000 people since 2011.

Obama and Merkel urged parties on the ground to "continue respecting the ceasefire, begin the disengagement of forces along the line of contact as quickly as possible and provide international monitors unfettered access to the entire conflict zone."

"The president and chancellor strongly condemned the barbarous Russian and Syrian regime airstrikes against eastern Aleppo," the White House said.

Two of the largest hospitals in the city's east were bombed on Wednesday in what UN Secretary-General Ban Ki-moon described as a war crime.

Air strikes pounded Aleppo province on Thursday while at least 11 civilians, including seven children, died during attacks on the city of Idlib, nearby Jarjanaz and central Hama province, the Syrian Observatory for Human Rights said.