Beirut: The Britain-based Syrian Observatory for Human Rights said IS militants killed at least 35 Syrian soldiers and pro-government forces in an attack on several areas in the city of Deir Al Zor on Saturday.
"The fighters have infiltrated the northwestern side of the city and carried out the attacks," said the Observatory which monitors the conflict in Syria via a wide network of local sources.
IS 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 IS'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 IS militants for more than a year and more than 200,000 people there are living in dire conditions lacking food and medicine.
A 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. There are a number of civilians martyred".
He said that the army killed large numbers of the attackers.
IS supporters on social media said the group had also captured an army weapons depot.
Reuters was unable to independently verify these reports.
Meanwhile, the United Nations has unverified reports that 15-20 people died of starvation in the Syrian city of Deir Al Zor in 2015, where 200,000 people face sharply deteriorating conditions and a severe food shortage, it said in a report published on Saturday.
There have been no flights into the airport except by helicopter since September, no electricity for more than 10 months and water is only available for three hours a week, the UN report said.