Islamic State fighters have seized a key gas field in central Syrian, a monitoring group and the governor of Homs said on Thursday.

According to the UK-based Syrian Observatory for Human Rights, IS fighters attacked the Shaar gas field on Thursday morning, killing 23 guards.

Palmyra itself has largely remained a government strong hold in the vast largely desert dominated central and eastern Syria.

"The fate of 340 National Defence Force (paramilitary) members, guards, engineers and employees who were in the field, is unknown, as they were either taken prisoner, wounded or captured during the operation," said Observatory director Rami Abdel Rahman.

The operation, he added, was the Islamic State's "most important so far against the government" of Syria's President Bashar al-Assad.

Abu Bilal, an activist linked to IS in Homs province, also reported the attack.

Twelve members of IS had been slain in the fighting, he said, adding that "dozens" were killed on the government side.

"There was a martyrdom [suicide] attack, and we took eight checkpoints before taking over the gas field," Abu Bilal told AFP.

Homs governor Talal Barazi also confirmed the attack telling AFP that the authorities “had lost contact with three technicians who were on the site."

"The armed men were present in the area beforehand, but they have now expanded their area of control with this new operation," he said.

"The army is trying to take it back. There is fighting in the area and government air strikes."

The news comes a day after Syrian president Bashar al-Assad was sworn in for another seven year term, despite the ongoing crisis in the country.

It also comes as four people were confirmed killed in Damascus following a mortar attack on Thursday. A further 34 people were injured, making the attacks one of the bloodiest on the capital in recent weeks.

No one immediately took responsibility for the incident, although Assad’s forces are in principle battling various rebel groups, as well as the IS.

IS State, which proclaimed a "caliphate" straddling Syria and Iraq last month, has in recent weeks taken over the entire countryside of the Deir Ezzor province, a sparsely populated but energy-rich region.

Deir Ezzor borders Homs province as well as Iraq, where the group has spearheaded a major Sunni militant offensive that has seen large swathes of territory fall out of the Iraqi government's control.

IS, however, is also reportedly selling oil and gas to Assad's government through secret channels.

"IS sells oil and gas to the [Syrian] regime through businessmen. There is no direct dealing between IS and the regime, but there is a lot of proof that these channels exist," Free Syrian Army spokesman Omar Abu Leyla told AFP earlier this month.

His views were held by French foreign minister Laurent Fabius at the end of June.

"We have proofs that when ISIL has taken over oil it has sold oil to the [Bashar al-Assad] regime," he said at a news conference.

“Officially they are combating each other but in fact they are very often helping each other," he added.