Syria's regime has accused armed groups of carrying out a “toxic gas” attack that left dozens of people struggling to breathe and prompted government ally Russia to launch retaliatory air strikes on Sunday.

Around 100 Syrians were hospitalised with breathing difficulties after the alleged chemical attack in the regime-held city of Aleppo on Saturday, state media and a monitor said.

Russia accused militants of being behind the alleged chlorine attack, and carried out the first air raids in months on the outskirts of a major rebel bastion west of the city.

It was the latest accusation of a chemical attack in Syria's grinding seven-year civil war, which has killed more than 360,000 people and displaced millions.

State news agency SANA reported “107 cases of breathing difficulties”, after what health official Ziad Hajj Taha said was a “probable” chlorine attack on Aleppo city.

The Syrian Observatory for Human Rights, a Britain-based monitor, said a total of 94 people were hospitalised, but most had been discharged, after “the smell of chlorine” was reported in the city.

On Saturday, an AFP photographer saw dozens of civilians, including women and children, stream into an Aleppo hospital, some on stretchers or carried in by their relatives.

The injured seemed to be dizzy and breathing with difficulty.

Staff gave them oxygen masks, through which they breathed for 15-minute sessions, either sitting or lying down.

Rebel denial

The regime controls Aleppo city, but rebels and militants are present to the west in the country's last major opposition bastion of Idlib.

On Sunday, Russian air raids hit a planned buffer zone on the edges of that stronghold, the Observatory and Moscow said.

They were the first air strikes to hit the expected demilitarised area since a September deal between Moscow and rebel backer Ankara to protect Idlib from a massive regime assault.

On Sunday, Moscow accused Hayat Tahrir al-Sham (HTS), an alliance led by militants of Syria's former Al-Qaeda affiliate, of carrying out the alleged toxic attack in Aleppo.

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

Official media accused “terrorists” of carrying out a “toxic gas” attack on Aleppo, using a term it uses to mean both rebels and militants.

A rebel coalition on Sunday denied any involvement.

“We at the National Liberation Front deny the criminal, lying regime's allegations that revolutionaries targeted the city of Aleppo with any missiles and especially not any containing chlorine gas,” it said.

Neither HTS nor the Al-Qaeda-linked Hurras al-Deen group, both of which are present near Aleppo, commented on the accusations.

'Pretext'

The Russia-Turkey buffer zone deal on September 17 intended to prevent a major regime attack on Idlib, which is home to some three million people.

But its implementation has stalled after militants refused to withdraw from the planned demilitarised zone on time, and sporadic shelling and clashes have rocked the area.

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

After the alleged toxic attack, Nasr al-Hariri, who heads Syria's mainstream opposition, accused Damascus of seeking a "pretext to launch a military offensive in northern Syria".

Over the course of Syria's war, international human rights groups have repeatedly accused the regime of carrying out chemical attacks.

Last year and in April, the allegations caused the United States to carry out punitive strikes in Syria.

A joint commission of the United Nations and the Organisation for the Prohibition of Chemical Weapons (OPCW) has also accused the militant Islamic State group of using toxic gases.

The Damascus regime on Sunday called on the UN Security Council to denounce the attack.

“The Syrian government demands the UN Security Council immediately and harshly condemn these terrorist crimes,” the foreign ministry said.

Since starting with the brutal repression of anti-government protests in 2011, Syria's civil war has spiralled into a complex conflict involving world powers and militants.