Containers of toxic gas have been dropped on a Syrian town overnight close to where a Russian military helicopter was shot down, a rescue service within the country ahs claimed.

Up to 33 people, mostly women and children, were affected by the suspected gas attack in Saraqeb, a spokesman for Syria Civil Defence told Reuters.

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 after being given an oxygen mask by people in civil defence uniforms.

Syria Civil Defence workers, who went to the scene of the attack, said they suspected it was chlorine but could not verify the type of gas used.

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

The suspected attack comes almost a year after the UN security council adopted a resolution that set a 12 month-deadline to identify the perpetrators of chlorine attacks in Syria. The deadline expires next week.

Western powers have said the Syrian government has been responsible for chlorine and other chemical attacks, while the government and Russia have accused rebel forces of using poison gas.

Both government and opposition forces have denied using chemical weapons during the five-year civil war.

But that did not stop the accusations from flying on Tuesday – with the opposition Syrian National Coalition (SNC) accusing President Bashar al-Assad of being behind the attack.

Later, state news agency SANA said rebels had fired rockets armed with toxic gas on the government-held old quarter of Aleppo city, killing five people and causing eight breathing difficulties. It gave no further details.

The SNC said of the reported use of poison gas in Saraqeb: “After shelling, besieging and killing civilians and perpetrating war crimes on them, the Assad regime has resorted once again, and in breach of UN resolutions 2118 and 2235, to using chemical substances and toxic gases.

“The daily reality confirms that all the international agreements and previous security council decisions, be they about chemical weapons or otherwise, are meaningless for the Assad regime.”

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

Syrian rebels gather around the wreckage of a Russian Mi-8 helicopter that was shot down on Monday (MOHAMED AL-BAKOUR/AFP/Getty)

Syrian rebels gather around the wreckage of a Russian Mi-8 military transport helicopter after it was shot down along the administrative border between Idlib province, northwestern Syria and neighbouring Aleppo (MOHAMED AL-BAKOUR/AFP/Getty Images)

Russia's defence ministry said a Russian helicopter was shot down near Saraqeb, killing all five people on board in the largest loss of life for the Russian military since it started operations in Syria's civil war 10 months ago.

The Mi-8 transport helicopter was brought down by ground fire in Idlib province while returning to Hemeimeem air base after delivering humanitarian goods to Aleppo, the ministry said. It said all three crew members and two military officers on board died.

Images uploaded to social media purportedly show the flaming wreckage of the helicopter along with a dead body and what are alleged to be Russian identity documents taken from the wreckage.

The helicopter appeared to have broken up as it crashed: its tail could be seen lying separately from the aircraft's body in flames.

Syria's civil war: Russian chopper shot down in Idlib

A rocket pod was visible amid the wreckage, standard equipment for the Mi-8, which can be used for carrying troops and cargo and attacking ground targets.

People standing nearby are seen taking photos on their phones and shouting “Allahu Akbar,” or “God is great” in Arabic. The body of one Russian soldier was seen being dragged by the legs while a man jumped on the half-naked body of another soldier.

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There was no immediate claim of responsibility for the attack and Idlib province has a strong presence of fighters from both Jabhat al-Nusra and other groups fighting Syrian President Bashar al-Assad's forces.

Rebels fighting for Jaysh al-Fatah (Army of Conquest) claimed to have recovered the body of the pilot on social media.