There was no letup in fighting between Syrian allied forces backed by Russia airstrikes and rebels in Aleppo on Thursday, a day after reports of a chlorine attack on rebel positions.

The Organization for the Prohibition of Chemical Weapons said Thursday the reports of possible chemical weapons use in Syria were "of great concern."

Opposing sides in the conflict have accused each other of using chlorine gas throughout the war.

Syrian rebels and activists said one Syrian government chlorine barrel bomb on the city's eastern Zabadieh neighborhood killed at least two people and sickened others on Wednesday.

Staffan de Mistura, the UN envoy for Syria, said Thursday that if the chorine gas attack took place it would constitute a "war crime."

"It's really not for me to assess who did it and whether it actually took place, although there is a lot of evidence that it actually did take place," de Mistura told reporters on Thursday.

French Foreign Minister Jean-Marc Ayrault said he was "concerned by reports of a new chemical attack... that is said to have claimed four lives and left dozens injured."

Rebels and jihadists broke a government siege over the weekend, allowing aid into Syria's second city.

Russia said Thursday it would consider extending a three-hour a day humanitarian ceasefire around Aleppo, but all sides of the conflict continued clashes on the ground.

Responding to Russia's offer, de Mistura said any pause in fighting was welcome but "three hours is not enough."

cw/xx (AFP, AP, dpa, Reuters)