The Observatory reported 21 cases of suffocation in Eastern Ghouta on 22 January. Residents and medical sources said they suspected a chlorine attack.
Syrian regime forces were engaged in heavy fighting with rebels in southern areas of opposition-held Eastern Ghouta on Sunday, a day after a UN call for a ceasefire
The two main groups controlling Syria's rebel enclave of Eastern Ghouta have welcomed a UN Security Council demand for a ceasefire, but vowed to fight back in case of renewed attacks.
Regime air strikes and artillery have been pounding the enclave near Damascus since February 18, with at least 519 dead since the bombing campaign was launched.
More than 120 children have been among the dead in the bombing campaign that the regime launched last Sunday on the enclave just outside Damascus.
UN Security Council postponed the vote, for a month-long ceasefire, until 1700 GMT on Saturday as Western powers bickered with Russia over the wording.
Heavy Syrian bombardment of the rebel-held enclave of Eastern Ghouta is targeting civilians and must stop immediately, the United Nations warned today.