Syrian regime air strikes killed at least 30 civilians today in the last rebel pocket in the former opposition bastion of Eastern Ghouta near Damascus, a war monitor said.

Eight children were among the dead in the town of Douma, according to the updated toll from the Britain-based Syrian Observatory for Human Rights, which relies on sources on the ground.

Earlier, Syrian rebel group Jaish al-Islam accused government forces of dropping a barrel bomb containing poisonous chemicals on civilians in Eastern Ghouta, injuring more than 500 people.

"The Assad regime and its allies carry on with their crimes," the rebels' military spokesman Hamza Birqdar told al-Hadath TV.

The Syrian Observatory for Human Rights reported 11 cases of symptoms of suffocation in the city, including five children, following strikes by Syrian warplanes. It did not say what agents may have been used in the attacks.

Syrian state media denied launching chemical attacks as soon as the reports began circulating.

"Jaish al-Islam terrorists are in a state of collapse and their media outlets are (making) chemical attack fabrications in an exposed and failed attempt to obstruct advances by the Syrian arab army," state news agency SANA reported, citing an official source.

The government launched a fierce air and ground assault on Douma, the last rebel-held town in Eastern Ghouta, yesterday, killing 48 people in the last 24 hours alone.

State TV showed thick clouds of smoke rising from Douma, where Jaish al-Islam is holding out after insurgents in other parts of Eastern Ghouta accepted offers of safe passage to rebel-held areas in the north of the country.

Six civilians were killed today in mortar shelling of residential neighborhoods of the capital Damascus, and 38 others were injured, Syrian state media reported, accusing Jaish al-Islam of the attacks. The rebel group's spokesman Birqdar issued a statement denying that accusation.