Islamic state militants killed dozens of people execution style in attacks on government-held areas in Syria's city of Deir al-Zor on Saturday, a source and a monitoring said.

A source close to the Syrian government side said the Islamic State fighters killed at least 250 people, including pro-government fighters and their families when they attacked the neighbourhoods of Begayliya and Ayash in the city.

He said some of the casualties were beheaded.

The Syrian Observatory for Human Rights, which monitors the violence in the country through a wide network of local sources, said the militants killed dozens. "We have 60 people confirmed killed, but the number is big. The details are hard to get so far but the deaths are in dozens," the Observatory's head Rami Abdulrahman told Reuters.

The Observatory said earlier that at least 35 soldiers and pro-government fighters were killed in the attacks.

A Syrian source said the group "committed a massacre among the civilians."

He said the army repelled the attacks and killed a large number of the attackers.

"They sent six suicide bombers first and they tried to break into military positions but they failed."

The Observatory said that the hardline group fighters had infiltrated the northwestern side of the city and carried out several attacks.

Islamic State group is in control of most of the eastern province while the government is holding parts of the city including a military airport.

Deir al-Zor province links Islamic State's de facto capital in Raqqa with territory controlled by the group in Iraq.

Government-held areas in the city had been under siege by Islamic State fighters for more than a year and more than 200,000 people there are living in dire conditions lacking food and medicine.

The Syrian source said that the group has been trying to attack the city almost on daily basis and on Saturday it carried out several assaults."

Islamic state supporters on social media said the group had also captured an army weapons depot and seized tanks.

Reuters was unable to independently verify these reports.

The Observatory also said that unknown warplanes carried eight air strikes on the city of Raqqa killing at least 16 people and wounded 30 others. – Reuters