Air strikes hit the edges of Syria's last major rebel stronghold west of Aleppo on Sunday, a monitor said, a day after an alleged toxic attack on the regime-held city.

They were the first to hit the area since Moscow and rebel backer Ankara agreed to set up the demilitarised area around Idlib in September to prevent a massive regime attack to retake the area.

The attack came after Russia seemed to accuse Hayat Tahrir al-Sham, an alliance led by Syria's former Al-Qaeda affiliate, of carrying out Saturday's chemical attack on Aleppo city.

Russian defence ministry spokesman Igor Konashenkov said "terrorist groups" in an area of buffer zone held by HTS fired shells filled with chlorine on a residential area of the city.

The Observatory said the attack left around 100 struggling to breathe, though more than half had been discharged by Sunday morning.

Syria's state news agency SANA reported "107 cases of breathing difficulties".

A rebel alliance denied any involvement, but HTS did not immediately issue a statement.

The September 17 deal was intended to protect three million inhabitants in the Idlib region, more than half of which is held by HTS.

But its implementation has stalled after jihadists including HTS failed to withdraw from the planned buffer zone by a mid-October deadline.

Syria's regime has insisted that the buffer zone deal is temporary and that Idlib will eventually revert to government control.

Syria's war has killed more than 360,000 people since it erupted in 2011 with the brutal repression of anti-government protests.

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