DAMASCUS • Syria's army has broken a years-long siege by the Islamic State in Iraq and Syria (ISIS) of the government enclave of Deir Ezzor city as it battled to expel the militants from a key stronghold.

Syria's army and allied fighters, backed by Russian air support, have been advancing towards Deir Ezzor on several fronts in recent weeks, and arrived in the Brigade 137 base on its western edge yesterday.

"The Syrian Arab Army this afternoon broke the siege of Deir Ezzor city after its advancing forces arrived from the western province to Brigade 137," state news agency Sana said.

State media reported celebrations inside the government-held parts of the city as the troops arrived.

Early this year, the government- held parts of the city were cut in two by an ISIS offensive. The army's advance to the base broke the siege on the northern part of the city, to which it is connected by a road.

But a southern government-held section, including the key military airport, remains surrounded.

Government forces are heading towards the southern sector, however, and are about 15km away, according to the Syrian Observatory for Human Rights, a Britain-based monitor.

Around 100,000 people are believed to be inside government- held areas of Deir Ezzor, with perhaps 10,000 more in parts of the city held by ISIS.

Syrian President Bashar al-Assad congratulated government troops who broke the siege, his office said.

"Today you stood side by side with your comrades who came to your rescue and fought the hardest battles to break the siege of the city," Mr Assad said in a phone call with the commanders of troops who had been besieged in a base in Deir Ezzor.

ISIS has lost more than half of its nearby bastion of Raqa to US- backed forces, and losing Deir Ezzor city and the surrounding oil- rich province would leave it with only a handful of isolated outposts.

The Russian Defence Ministry said its warship yesterday fired cruise missiles from the Mediterranean at the ISIS group near Deir Ezzor. The strike, which was carried out by the frigate Admiral Essen, hit ISIS targets near the town of Al-Shula, which were controlled by fighters from Russia and other post-Soviet nations.

As a result of the strike, a "large" group of fighters was destroyed, along with a communications centre, warehouses and ammunition, the ministry said in a statement.

"The launch of the Kalibr cruise missiles has ensured the continued success of the advance by Syrian government troops and helped thwart plans of ISIS militants to regroup and reinforce the positions of terrorists in the vicinity of Deir Ezzor," the ministry said.

Syria's army still faces a potentially difficult battle to break the siege in the south of the city and free its remaining neighbourhoods, and the surrounding province, from ISIS. But for the government, its success would be "one of the most symbolic victories in its six-year war", wrote Syria analyst Aron Lund in a recent analysis.

"The reopening of the Deir Ezzor road is a strategic disaster for ISIS, which is now at its weakest since 2014 and seems unable to break out of an accelerating spiral of defeats," he added.

Government forces and tens of thousands of civilians in the city have been trapped for over two years, facing food and medical shortages.

AGENCE FRANCE-PRESSE