Barrels of suspected chlorine gas have been dropped by helicopter on a town in northern Syria, rescuers have said.
The gas was dropped overnight on a town close to where a Russian military helicopter was shot down hours earlier, the rescuers said.
A spokesman for the Syria Civil Defence said 33 people, mostly women and children, were affected by the chlorine.
The attack happened in Saraqeb, in rebel-held Idlib province.
Burning wreckage of the Russian helicopter
The opposition Syrian National Coalition (SNC) accused President Bashar al Assad of being behind the attack.
Mr Assad has denied previous accusations of using chemical weapons.
:: British Man Killed in Syria ‘Fighting IS’
Syria Civil Defence, which describes itself as a neutral band of search and rescue volunteers, posted a video on YouTube.
It shows a number of men struggling to breathe and being given oxygen masks by people in civil defence uniforms.
“Medium-sized barrels fell containing toxic gasses,” a spokesman for the group said.
“The Syrian Civil Defence was not able to determine the type of the gas,” the spokesman added.
The Syrian government and its Russian allies were not immediately available for comment.
The SNC said in a statement: “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 gasses.
“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.”
The Civil Defence spokesman said it was the second time Saraqeb had been hit by toxic gas.
The group was aware of around nine suspected chlorine gas incidents across Idlib province since the conflict began, he said.
Monitors at the Britain-based Syrian Observatory for Human Rights said barrel bombs fell on Saraqeb late on Monday, wounding many citizens.
Russia’s defence ministry said a Russian helicopter was shot down near Saraqeb during the day on Monday.
The crash killed all five people on board, in the biggest loss of life for Russian forces since they started operations in Syria.
Separately, Russia launched heavy air strikes overnight on the outskirts of divided Aleppo.
Aleppo has been devastated by the ongoing war
The strikes were aimed at slowing a “last-chance” assault by rebels seeking to break a government siege of the city.
The assault began on Sunday and is intended to ease the encirclement of the opposition-held east of the city.
An estimated 250,000 residents have been under a regime siege since 17 July.
The British-based Syrian Observatory for Human Rights described it as the largest rebel attack in Aleppo since 2012.
Government troops backed by Russia’s air force have put up a fierce defence, leaving many dead.