Syrian government forces repelled an attack by the Islamic State group on a major air base in the country's east on Saturday, in fighting that killed dozens on both sides, activists and state media said.

The base, outside the city of Deir el-Zour, has been used by the government in the past months to launch air raids on areas along Syria's border with Iraq that are held by the Islamic State group, also known by its initials ISIS and ISIL. 

ISIS has been trying to capture the air base and a nearby barracks known as Brigade 137 to eliminate the main pocket of resistance in the area and provide a major morale and propaganda boost after a string of setbacks in recent weeks.

Syria airstrikes by Assad forces

People run amid smoke from what activists said were last week's airstrikes by Assad forces in eastern Syria. (Nour Fourat/Reuters)

The Britain-based Syrian Observatory for Human Rights said the latest attack began Saturday with a suicide car bomb at the main entrance of the air base. The rebels stormed parts of the base before a counterattack and intense air raids by government forces pushed them back, it added.

The group said that some ISIS fighters had breathing problems in the area after government forces used chlorine gas against them.

Syria was supposed to have surrendered its entire arsenal of chemical weapons by last April as part of an international agreement, but the list of chemical munitions the country submitted to inspectors didn't include stockpiles of chlorine. Military use of chlorine gas to kill or harm violates the Chemical Weapons Convention, which Syria acceded to last year.    

Syrian state TV quoted an unnamed military official saying that troops repelled attempts by "terrorists" on several areas near the city of Deir el-Zour and killed "tens of them and destroyed their vehicles and weapons."

The station later aired footage of Syrian troops standing near bodies of dead fighters it said were ISIS members who were killed in Deir el-Zour.

Activists claim helicopters captured

ISIS began a major offensive on the air base, one of the last government-held areas in the province, on Thursday.

The rights group said that since then, more than 150 fighters on both sides have been killed, including 51 troops and pro-government militiamen. It said at least 100 ISIS fighters have been killed, including two French citizens.

Activists who support the Islamic State group posted photos on social media showing two military helicopters that they claimed were captured by the jihadis.

On Friday, Islamic State group fighters captured the strategic village of Jafra near the air base. 

With files from CBC News