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

A spokesman for Syria Civil Defense told Reuters that 33 people, mostly women and children, were affected by the gas in Saraqeb.

The group, which describes itself as a neutral band of search and rescue volunteers, posted a video on YouTube purportedly showing a number of men struggling to breathe and being given oxygen masks by people in civil defense uniforms.

Syria Civil Defense workers, who went to the scene of the attack, said they suspected it was chlorine but could not verify that.

"Medium-sized barrels fell containing toxic gasses. The Syrian Civil Defense was not able to determine the type of the gas," said the spokesman.

The Syrian regime and its Russian allies were not immediately available for comment.

Western powers say the regime has been responsible for chlorine and other chemical attacks.

Monitors at the Syrian Observatory for Human Rights said barrel bombs fell on Saraqeb late on Monday, injuring a large number of citizens.

Russia's defense ministry said a Russian helicopter was shot down near Saraqeb on Monday, killing all five people on board, in the biggest officially acknowledged loss of life for Russian forces since they started operations in Syria.

The helicopter came down in Idlib province, roughly mid-way between Aleppo and the Russian air force base at Khmeimim, near the Mediterranean coast.

PHOTO CAPTION

People inspect the wreckage of a Russian helicopter that had been shot down in the north of Syria's opposition-held Idlib province, Syria August 1, 2016.

Reuters