Fighters from the Islamic State of Iraq and the Levant (ISIL) group have overrun a government-controlled hospital in eastern Syria, killing 20 members of pro-government forces and taking medical staff hostage, a UK-based monitoring group said.

The Syrian Observatory for Human Rights said on Saturday the fighters attacked Al-Assad hospital in Deir Az-Zor as they press an advance to control the oil-rich city and its vital airbase.

The attack sparked clashes with government forces providing security for the hospital in which six fighters were killed, the monitor said.

"ISIL attacked Al-Assad hospital at the city's western entrance, killing at least 20 soldiers and allied fighters," said Observatory head Rami Abdel Rahman.

The fighters "seized the hospital and captured the medical staff, holding them hostage", he said, adding that the fighting was still raging.

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ISIL's Amaq news agency said that the group also took control of a check point, a fire station and university accommodation in the city close to Syria's eastern border with Iraq.

Amaq also said the fighters had taken territory near to the state-held military airport.

ISIL controls about 60 percent of Deir Az-Zor, including the centre and the north of the city.

It has imposed a siege on government-held districts in the south and the east where about 200,000 civilians have been trapped since March 2014.

The fighters, who also control nearly all the surrounding province, have repeatedly attacked the government enclave and seized several neighbourhoods since the start of this year.

But their efforts to capture the airbase located in the south of the city have been crushed by government troops.