Russia's foreign ministry has sharply criticised US behaviour in Syria, accusing Washington of backing rebels who use poison gas against civilians and of killing hundreds in air strikes.

The ministry's spokeswoman, Maria Zakharova, on Thursday referred to rebels supported by Washington as "animals" and said Washington and its allies were recklessly carrying out air attacks leading to numerous civilian deaths.

Zakharova's strong comments, in a statement posted to social media, underscored the serious divisions between Moscow and Washington over Syria where they are united in their opposition to Islamic State but so far little else.

Moscow backs President Bashar al-Assad and his forces, while Washington supports rebel groups fighting both his government and Islamic State militants. While the two countries argue over a possible peace plan, a battle for eastern Aleppo has erupted.

Zakharova spoke out as the United Nations said the United States and Russia were in intensive discussions to shore up Syria's collapsed nationwide truce with their military experts trying to agree a cooperation plan "that would unlock the entire solution."

Zakharova criticised the United States over an incident the Russian military said occurred on August 2 in eastern Aleppo when rebels used poison gas, killing at least seven people.

She blamed the Free Syrian Army's Nour al Din al-Zinki group, which has received US military backing, and said last month it was investigating the beheading of a young child in Aleppo after video footage circulated showing the boy being killed by a man whom activists identified as a member of the group.

"The United States is supporting these animals who used poison gas against the civilian population," Zakharova wrote.

"Unfortunately it's not the only tragedy which the 'moderates' backed by Washington stand behind."

She accused the US air force of carelessly bombing inhabited parts of Manbij in northern Syria where US-backed forces - the Syria Democratic Forces (SDF) - are taking on Islamic State militants.

"The United States and the SDF are not taking any steps to warn people to avoid deaths," she said, saying that U.S and coalition aircraft had killed hundreds and wounded thousands of civilians according to "conservative estimates."

"If our Western colleagues and above all Washington do everything again not to notice these facts then any of their talk about an end to bloodshed in Syria will simply become preposterous."