The world's chemical watchdog said Monday (November 26th) it was reviewing security ahead of a possible probe into an alleged chemical attack in Syria's regime-held city of Aleppo over the weekend, AFP reported.

Damascus formally requested that the Organisation for the Prohibition of Chemical Weapons (OPCW) investigate the alleged Saturday attack, which Syrian officials and rights groups said left scores of people struggling to breathe.

Both the Syrian regime and Russia have blamed "terrorist groups" -- a term Damascus uses to mean both mainstream opposition fighters and extremists.

The alleged attack also prompted Russia to launch retaliatory airstrikes into a planned buffer zone near the last major opposition stronghold of Idlib.

"The OPCW secretariat has been monitoring the situation," said OPCW director-general Fernando Arias, and has contacted the UN's security department "in order to assess the security situation on the ground for a possible deployment of the Fact-Finding Mission (FFM) to Syria".

The secretariat implements the OPCW's verification measures while the FFM, set up in 2014, investigates all allegations of chemical weapons use in Syria.

"The OPCW's experts will continue to work independently to verify all allegations of the use of chemicals as weapons in Syria," Arias said.

Syria on Monday blamed "armed terrorist groups" for launching a toxic gas attack which it said left around 100 Syrians with breathing difficulties.

"It is believed that the agent used was chlorine," said Bassam al-Sabbagh, Syria's permanent representative to the OPCW.

Russia said the shelling came from an area of the buffer zone controlled by Tahrir al-Sham.

An opposition coalition has denied involvement, but neither Tahrir al-Sham nor al-Qaeda-linked Hurras al-Din have commented on Saturday's alleged attack.