BEIRUT (Reuters) - Syrian government forces retook a hospital in Deir al-Zor after Islamic State attacked it on Saturday following a dawn offensive by the militants on the besieged eastern Syrian city, a war monitor and state media said.

An Islamic State flag is seen in this picture illustration taken February 18, 2016. Picture taken February 18, 2016. REUTERS/Dado Ruvic/Illustration

The British-based Syrian Observatory for Human Rights, which monitors Syria’s five-year-long war, said Islamic State had killed at least 35 members of the Syrian armed forces and detained some medical staff from the hospital.

The fighting in and around the hospital also killed at least 24 Islamic State fighters, the Observatory said.

Islamic State’s Amaq news agency said its fighters staged a “major offensive” on the southwestern edge of Deir al-Zor on Saturday, storming the Assad Hospital and cutting the supply route between the a Syrian army base and the airport.

Islamic State controls most of Deir al-Zor province and has laid siege since March last year to the remaining government-held areas in the city of the same name, which is close to Syria’s eastern border with Iraq.

Deir al-Zor province links Islamic State’s de facto capital in the Syrian city of Raqqa with territory controlled by the militant group in neighboring Iraq.

Islamic State also said it had taken control of a check point, a fire station, university accommodation, grain silos and some territory near the al-Tayyam oil fields, in the vicinity of the state-held military airport on the city’s southern edge.

Russia’s RIA state news agency on Saturday reported a source within the airbase on the southern edge of the city as saying an Islamic State attack had been repelled.

The Observatory and Amaq said there were ongoing fierce clashes between government forces and Islamic State in the area of the attack.

Islamic State said it killed at least 80 government troops, took three prisoner and destroyed a number of armored vehicles.

The Syrian government and its Russian allies make regular aid drops into the encircled city and there are frequent air strikes on Islamic State targets in and around Deir al-Zor.