MOSCOW (Reuters) - Russia’s foreign ministry sharply criticized U.S. behavior in Syria on Thursday, accusing Washington of backing rebels who use poison gas against civilians and of killing hundreds in air strikes.

Spokeswoman of the Russian Foreign Ministry Maria Zakharova attends a news briefing in Moscow, Russia, October 6, 2015. REUTERS/Maxim Shemetov

The ministry’s spokeswoman, Maria Zakharova, 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 criticized the United States over an incident the Russian military said occurred on Aug. 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 for what she said was a crime. The same group, which has received U.S. military backing, 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 U.S. air force of carelessly bombing inhabited parts of Manbij in northern Syria where U.S.-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.”

She also referenced an incident on July 28 in which she said U.S.-led coalition air strikes had killed 28, including seven children, in attacks on the village of al Ghandour in the countryside north of Manbij city.

The U.S. military said last month it was investigating that episode.

“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,” wrote Zakharova.

Moscow is itself regularly accused by human rights groups and the United States of carrying out deadly air strikes on non-military targets and of siding with an army accused of abuses. It says it takes great care to avoid civilian deaths.

A Syrian rescue service operating in rebel-held territory said on Tuesday that a helicopter dropped containers of toxic gas overnight on a town close to where a Russian military helicopter had been shot down hours earlier.

The Kremlin said suggestions it was a revenge attack and that Russian forces or the Syrian army were responsible were false.