The Britain-based Syrian Observatory for Human Rights said Islamic State group fighters killed at least 35 Syrian soldiers and pro-government forces in an attack on several areas in the city of Deir Ezzor 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.
The terrorist organization, aka ISIS, is in control of most of the eastern province, while the government is holding parts of the city, including a military airport.
Deir Ezzor province links ISIS' de facto capital of Raqqa with territory controlled by the group in Iraq.
Government-held areas in the city have been under siege by ISIS fighters 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 the group has been trying to attack the city on almost a daily basis, and Saturday it "carried out several assaults. There are a number of civilians martyred."
He said that the army killed large numbers of the attackers.
ISIS supporters on social media said the group had also captured an army weapons depot.
Reuters was unable to independently verify these reports.