Syrian government forces have launched a counterattack to recapture a gas field seized by the Islamic State group, activists say, even as the death toll from three days of fighting there rises to 270.

The intense fighting in the Shaer field, which lies in the desert region of Palmyra in the central province of Homs, has been among the deadliest battles between government forces and the Islamic State group since the start of the Syrian uprising more than three years ago.

Al Jazeera cannot independently verify the Syrian Observatory's statement due to reporting restrictions inside Syria.

Fighters from the Sunni-led rebel group have in the past few weeks seized a huge chunk of territory straddling the Iraq-Syria border, where they declared a self-styled caliphate. They also have captured much of Syria's oil-rich eastern province of Deir Az Zor.

The Britain-based Syrian Observatory for Human Rights said Syrian special forces launched an attack late on Friday on the Shaer field and have regained parts of it.

A Homs-based activist who goes by the name of Beibares Tellawi confirmed that troops attacked fighters from the group.

"The fighting today is mostly hit-and-run attacks," Tellawi said via Skype. "Large numbers of [President Bashar] Assad's forces are attacking the field."

The Syrian Observatory said that the number of troops, guards and workers killed in the gas field since it was captured on Thursday has risen to 270, adding that some were captured and killed by Sunni rebels. Tellawi said the death toll was at least 200.

Workers' fate unknown

The Observatory said Friday's clashes alone left 51 soldiers either dead or wounded. It said at least 40 Islamic State fighters were killed so far, while Tellawi said the number was more than 30.

The fate of 90 other gas field workers and guards is still unknown, said the Observatory, which relies on a network of activists inside Syria.

In the rebel-held Damascus suburb of Douma, a car bomb exploded on Saturday, killing at least nine people and wounding others, the Observatory said.

The area has been struck by several car bombs in the past month that killed and wounded dozens.

The Syrian Observatory and Ahmad al-Masalmeh, an opposition activist in the southern province of Daraa, also reported intense shelling and air raids by government forces in the region that borders Jordan.

Syria's state news agency SANA said the army "hit terrorist dens", killing and wounding scores of fighters in several villages and towns in Daraa, including the villages of Tafas, Inkhil, Atman and Dael.

An unnamed Jordanian military official quoted by Jordan's state news agency said that border guards had received 411 Syrian refugees over the past three days. The official said 46 of them were seriously wounded and were admitted to field hospitals and that 12 had died.

The Syrian uprising began in Daraa in March 2011 with anti-government protests. The revolt later turned into a civil war that has killed at least 170,000 people and displaced up to nine million, a third of the country's pre-war population.

Also on Saturday, the UN children's agency said it was able to reach the Damascus suburb of Moadamiyeh for the first time since 2012.

Moadamiyeh has been under a government siege that activists say killed a number of people because of lack of food and medicine.

On Monday, about 13 Syrian Arab Red Crescent lorries loaded with 1,000 food parcels crossed into Moadamiyeh.

UNICEF estimates there are 9,200 children in the suburb who are in desperate need of aid.