The Islamic State militant group (Daesh) on Saturday launched one of its fiercest assaults yet on the besieged Syrian city of Deir Ezzor, leaving more than 30 people including regime fighters dead.

The brutal attack -- on a day that saw many outbreaks of violence across Syria -- came as the political opposition said it "supported" upcoming peace talks in the Kazakh capital Astana.

The negotiations will attempt to bring an end to the nearly six-year war by building on a fragile truce agreement.

But IS is excluded from the deal, brokered by rebel backer Turkey and regime ally Russia.

Unleashing a wave of suicide attacks, rockets, and tunnel bombs, IS killed at least 12 government forces and two civilians in Deir Ezzor, according to the Syrian Observatory for Human Rights.

The British-based monitoring group said 20 militants were killed in fierce air raids by Syrian and allied warplanes on the city, where around 200,000 people have lived under IS siege since early 2015.

IS has sought to overrun the entire city, including the key nearby military airport.

Observatory head Rami Abdel Rahman said the attack was the "most violent" assault mounted by IS on the city in more than a year.

"Daesh is amassing its forces to attack Deir Ezzor 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.

Source :AFP