Dozens of people, mostly women and children, have been killed and 80 others injured after Syrian government forces bombed an underground shelter in Eastern Ghouta, according to rescuers and activists on the ground.

Sources on the ground and rescuers from the Syrian Civil Defence, a volunteer rescue group also known as the White Helmets, said on Friday that at least 37 victims burned to death after air raids carrying napalm gas hit the shelter in the town of Irbin.

"An air strike targeted one of the underground cellars in Irbin last night where between 117 to 125 people, mostly women and children, were hiding," Izzet Muslimani, an activist in Eastern Ghouta, told Al Jazeera.

Abul Yusr, an activist and citizen journalist in Irbin told Al Jazeera that the death toll was 45, and that the air raid had hit two shelters connected to one another by a corridor, leading to one becoming completely destroyed.

"The air strike entered through one shelter, where it exploded and killed everyone in it. The fires spread through to the second shelter, which soon became totally engulfed in flames," Abul Yusr told Aljazeera.

"Some people managed to escape the fire in the second shelter, but their injuries are quite severe."

"The injured are still being treated for first and second-degree burns at a makeshift clinic," Muslimani said.

"With the lack of emergency services, we are expecting the death toll to rise."

Zaher Hassoun, another activist in the rebel enclave, confirmed reports that the Syrian government had hit the shelter with napalm gas, a flammable liquid that is used in warfare as it sticks to skin and causes severe burns.

According to Abul Yusr, the bodies had become completely charred when they were pulled out from the underground cellars. They were buried in a mass grave in Irbin on Friday, he added.

Earlier this month, the Syrian Civil Defence said the Syrian government hit Irbin with chlorine gas, phosphorous and napalm. The news followed reports of several alleged chemical attacks in a matter of days.

Eastern Ghouta has been under control of armed opposition groups since 2013 - two years into a popular uprising in Syria calling for the removal of President Bashar al-Assad.

More than 1,500 civilians have died in the enclave, east of the capital, Damascus, since the government forces backed by the Russian warplanes launched a fierce assault on February 18.

Evacuation ongoing

The attack reportedly came just hours before a ceasefire went into effect after midnight which saw hundreds of opposition fighters and their families evacuated on Thursday from the town of Harasta in the eastern part of Eastern Ghouta, according to Syrian state media.

Sana, the official Syrian news agency, said at least six buses carrying the evacuees were headed to Idlib province in Syria's north, which is an area under opposition control.

The evacuation comes as part of a deal brokered on Wednesday by the Syrian government's principal ally, Russia, between fighters in Harasta and a Syrian government delegation.

The operation was facilitated by a United Nations delegation as well as the Syrian Red Crescent.

Until recently, Eastern Ghouta was one of the last remaining rebel strongholds. In a renewed offensive by the government since February 18, the opposition lost large parts of it, with the Syrian army claiming it has now recaptured 80 percent of the suburb.