The jihadist group has already lost more than half of its nearby bastion of Raqa to attacking US-backed forces.
The jihadist group has already lost more than half of its nearby bastion of Raqa to attacking US-backed forces, and the loss of Deir Ezzor city and the surrounding oil-rich province would leave it with only a handful of isolated outposts.
Government forces and civilians have been under siege in Deir Ezzor city since early 2015, and the regime-held parts are divided into a northern and southern enclave.
On Tuesday, government troops and allied fighters backed by Russian air strikes were within two kilometres (1.2 miles) of the Brigade 137 base on the city's western edge, state television said.
They are seeking to break the siege imposed by the jihadists on regime forces at the military base, which is linked by a single road to the rest of the northern government-held enclave in Deir Ezzor.
"IS has received reinforcements in the area, most of them foreign fighters, and they are fighting to try to stop the army from reaching the forces inside the base," a security source on the ground told AFP.
"The army is using air strikes and heavy artillery in an intensive fashion. There is lots of coordination between the attacking army forces and the soldiers who are besieged" inside the base, he added.
In government-held parts of the city under IS siege, the sound of fighting and powerful explosions could be heard, according to a local journalist contributing to AFP.
In the area, where the siege of more than two years has caused food and medical shortages, the national flag was on display everywhere in preparation for celebrations to welcome arriving troops.
Some local residents were already greeting each other with "Good morning of victory," he added.
"At the moment, the regime forces have not managed to enter the Brigade 137 base because of the minefields" around it, said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights, a British-based monitor.
"Overnight and until dawn, IS launched multiple counterattacks, without succeeding in recovering the areas it has lost," he added.
"Combat is ongoing and air strikes are continuing," he said.
Syrian government forces are also approaching the southern regime-held enclave in the city, which includes a military airport and three districts and is also under IS siege.
More than 330,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.
The attack on Deir Ezzor comes after a US-backed force captured more than half of nearby Raqa city, which IS overran in 2014, turning it into the de facto Syrian capital of its self-proclaimed "caliphate".