IS unleashes 'violent' attack on Syria's besieged Deir az-Zour

Intense fighting erupted between Syrian troops and IS militants in the city and near a military airport controlled by government forces, leaving dozens dead.

Saturday's attack was the "most violent" assault mounted by IS on the city in more than a year, said head of the London-based Syrian Observatory for Human Rights Rami Abdel Rahman.

IS was using tunnel bombs and suicide attackers, while Syrian and allied warplanes battered militant positions with airstrikes, he added.

"Daesh is amassing its forces to attack Deir az-Zour and breach government lines," a Syrian military source told AFP, using the Arabic acronym for IS.

He said militants had aimed to cut the route between the airport and the city, but that the government's counter-attack had stopped IS. 

"Warplanes attacked Daesh's supply lines in all the battlefronts and around the airport," he said.

At least 12 members of government forces and 20 IS militants were among the dead in the IS attack on regime positions in Deir az-Zour, according to the Observatory.

Both the Observatory and Syrian state news agency SANA said two civilians were also killed in IS rocket fire on government-controlled zones in the city. 

The militants, who control most of Deir az-Zour province, have kept the provincial capital under siege since 2014, but government forces have so far withstood the encirclement thanks to air-dropped humanitarian assistance and weapons and ammunition flown into the airport.

On Monday, the IS blew up a natural gas plant that supplied one-third of Syria's electricity, one month after getting its hands on the facility.

The plant had already ceased to operate one month ago, after the advance of the militants in the central region of Palmyra. 

IS on Sunday released a video entitled "Hayyan Gas Company explosion in the east of Homs province" in which a man is seen planting explosives before a wide-angle shot of a huge explosion engulfing the plant.

Since 2014, IS has seized several gas and oil fields in Syria, especially in the central and eastern parts of the country.

The US-led coalition regularly carries out airstrikes on oil wells operated by the militants.