Toxic gas was dropped on a Syrian town near where a Russian helicopter was shot down early this week, a Syrian rescue service told the Reuters news agency on Tuesday.

Syria Civil Defence, which describes itself as neutral group of search and rescue volunteers, posted a video online which purportedly shows men, struggling to breathe, being given oxygen by people wearing the group's uniform, Reuters said.

"Medium-sized barrels fell containing toxic gasses. The Syrian Civil Defence was not able to determine the type of the gas," a spokesman for the group told Reuters, saying the group suspected chlorine gas had been used.

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33 mostly women and children were affected in the town of Saraqeb, the spokesman told Reuters.

The Syrian Observatory for Human Rights said that barrel bombs fell on Saraqeb late on Monday and injured a large number of citizens, according to Reuters.

The Russian Tass news agency reported that a Kremlin spokesman said the claims of chemical gas being used in Saraqeb were false.

"I don't have information; we don't have information about how the operation is proceeding," Kremlin spokesman Dmitry Peskov said, according to Tass.

"It is very hard to react to such information releases: it is not always clear what they are based on, what their sources are and so on," Tass reports Peskov said.

A Russian transport helicopter was shot down in Syria on Monday, and Russian President Vladimir Putin's spokesman said all five crew member on board were killed. It was the deadliest single incident for Russia since the country began air strikes in Syria.