The Islamic State group launched a ferocious assault Saturday on a city in eastern Syria, leaving more than 30 jihadists and regime fighters dead, a monitoring group said.

At least 12 members of government forces and 20 ISIS militants were among the dead in the attack on regime positions in Deir Ezzor, according to the Syrian Observatory for Human Rights.

Both the Observatory and Syrian state news agency SANA said two civilians were also killed in ISIS rocket fire on government-controlled zones in the city.

Around 200,000 people live in Deir Ezzor city, which has been besieged by ISIS since early 2015 and is the capital of the oil-rich province of the same name.

ISIS has sought to overrun the entire city, including the key military airport nearby.

Observatory head Rami Abdel Rahman said Saturday’s attack was the “most violent” on the city in more than a year.

ISIS was using tunnel bombs and suicide attackers, while Syrian and allied warplanes battered jihadist positions with air strikes.