Syrian government forces clashed with Islamic State group fighters for a fourth day on Tuesday in an eastern city where a jihadist assault has left more than 100 dead, a monitor said.
IS began a fierce attack Saturday on the remaining parts of Deir Ezzor under government control, and has managed to divide the city's eastern half from the west.
It has also cut the supply route running from the city's key military airport, limiting the government's ability to bring in supplies and military reinforcements.
The Britain-based Syrian Observatory for Human Rights monitor said on Tuesday that Syria's military and allied Russian forces were carrying out air strikes against IS, as government troops battled the jihadists on the ground.
IS already controlled more than half of Deir Ezzor city, and has laid siege to around 100,000 people inside since early 2015.
But the latest assault, which included waves of suicide bombers, is the "most violent" attack on the city in more than a year, according to the Observatory.
The monitor said that three days of fighting had killed at least 116 people, among them 21 civilians, 37 members of regime forces and 58 IS fighters.
It said the government was flying reinforcements into the military base and had called up local residents to fight on the front lines against IS, including some without military training.
Since the siege began, the government has been able to fly limited supplies into the airport, and international aid has been intermittently dropped into the city.
But residents have nonetheless faced shortages and rising prices, as well as being unable to leave the city.
Deir Ezzor sits in the oil-rich eastern province of the same name, most of which is controlled by IS.
The extremist group has lost swathes of territory in northern Syria to Kurdish fighters as well as a Turkish-backed rebel alliance, but it remains on the offensive in other parts of the country.
More than 310,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests that were met with a regime crackdown.
The violence has drawn in international players, as well as attracting jihadist groups like IS.