The Syrian Observatory for Human Rights said at least 60 people in the besieged rebel enclave were left struggling to breathe after air strikes and barrel bombs hit the towns of Saqba and Hammuriyeh.

Doctors at one medical facility in Eastern Ghouta said they treated at least 29 patients with signs of exposure to chlorine, according to the Syrian American Medical Society (SAMS).

SAMS is a medical charity that supports hospitals in Eastern Ghouta and other rebel-controlled zones in Syria.

It did not report any deaths but said it was likely that more victims were being treated at other clinics.

"Due to chlorine attack in #EastGhouta, patients are struggling w/symptoms such as severe dyspnea, sweating, congestion of mucus membranes, severe runny nose, wheezing & conjunctival erythema," SAMS wrote on social media late Wednesday.

"The emotional trauma from these attacks can not be measured."

Dyspnea is shortness of breath. Conjunctival erythema is redness of the eye caused by dilation of the blood vessels.

Eastern Ghouta is facing a blistering offensive by Russian-backed government forces and allied militiamen, who are seeking to clear out rebels from the capital's outskirts.

Regime forces have been repeatedly accused of using chlorine on Eastern Ghouta in recent weeks, which both the government and its ally Russia have staunchly denied.

United Nations investigators say government forces used chlorine as a weapon at least three times between 2014 and 2015, as well as sarin gas in 2016.

The latest assault on Eastern Ghouta began with a intense bombing campaign on February 18, followed by a ground offensive.

Government forces have since captured more than half of the rebel-held parts of the area and killed more than 890 civilians, according to the Britain-based Observatory.

The toll includes at least 87 people killed on Wednesday.

Air strikes continued to hit the remaining rebel-controlled parts of Ghouta on Thursday.