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 "terrorist groups" 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 after "the smell of chlorine" was reported in the city, but most were discharged.

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 jihadists 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 said "terrorist groups" based in that buffer zone carried out the alleged toxic attack in Aleppo.

Russian defence ministry spokesman Igor Konashenkov said they fired shells filled with chlorine on a residential area of Aleppo.

He said the shelling came from an area of the buffer zone controlled by the jihadist-dominated Hayat Tahrir al-Sham (HTS) alliance.

Syrian state media accused "terrorists" of carrying out a "toxic gas" attack on Aleppo, using a term it uses to mean both rebels and jihadists.

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 present in the area, have commented on the alleged attack.

- '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 jihadists refused to withdraw from the planned demilitarised zone on time, and sporadic shelling and clashes have rocked the area.

On Sunday, Turkey's defence minister and his Russian counterpart spoke on the phone about the "latest provocations" in the Idlib region, Ankara reported, without providing more details.

Syria's regime has insisted that the buffer zone 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 Islamic State group of using toxic weapons.

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.

Syria's war has evolved into a complex conflict including world powers and jihadists, since starting in 2011 with the brutal repression of anti-government protests.