Militants from the Islamic State have seized a gas field in central Syria, according to a UK-based watchdog.

The Syrian Observatory for Human Rights said that fighters from the Islamic State attacked the Shaar gas field, killing 23 guards in the process.

"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 Rami Abdel Rahman, director of the observatory, as quoted by AFP news agency.

The operation was the "most important so far against the government" of Bashar al-Assad, Rahman added.

Homs governor Talal Barazi confirmed the attack had taken place.

"Armed men took control on Wednesday evening of the gas field, and we have lost contact with three technicians who were on the site," Barazi told AFP.

"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 Islamic State has recently seized all the major oilfields in Syria's eastern province of Deir Ezzor, as it seeks to boost its revenue streams.

The group has also declared a 'caliphate', straddling parts of Iraq and Syria that they have captured in recent months.

Syrian President Bashar al-Assad was sworn in for a third term on Wednesday. He is facing an ongoing insurgency as Syrian rebels and foreign jihadists attempt to overthrow the Syrian leader.