Deir Ezzor (Syria) (AFP) - Syria's army broke a years-long Islamic State group siege on the government enclave of Deir Ezzor city on Tuesday as it battles to expel the jihadists from a key stronghold.

The jihadist group has already lost more than half of its nearby bastion of Raqa to US-backed forces, and its expulsion from Deir Ezzor city and the surrounding oil-rich province of the same name would leave it with only a handful of isolated outposts.

Syria's army and allied fighters, backed by Russian air support, have been advancing towards Deir Ezzor on several fronts in recent weeks, and on Tuesday entered the Brigade 137 base on its western edge, in what Moscow hailed as a key "strategic victory".

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

"This great achievement is a strategic shift in the war on terror," the army command said.

A local journalist contributing to AFP on the ground said a minesweeper moved ahead of troops as they arrived at the base.

As they reached soldiers who had been besieged inside the base and adjacent parts of the city, the troops embraced and shouted patriotic slogans.

Others fired in the air and flashed victory signs, as Syrian and Russian warplanes flew overhead.

"We promised that we would not let Deir Ezzor fall, and it did not fall," General Issam Zahreddine, commander of the 7,000 soldiers in the city, shouted jubilantly to journalists.

Civilians gathered on either side of the road connecting the base to neighbourhoods of the city to welcome the arriving troops.

President Bashar al-Assad congratulated troops in a call to commanders at the base, 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 on the city," he said.

And Russia hailed the breaking of the siege as a "very important strategic victory", with President Vladimir Putin congratulating his country's troops in Syria and the government forces.

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A source in the Deir Ezzor governorate said trucks loaded with food and medicine were expected to arrive inside the besieged city from Aleppo within hours.

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

Early this year, the government-held parts of the city were cut in two by an IS offensive.

The army's advance Tuesday broke the siege on the northern part of the city, but a southern section, which includes a key military airport, remains surrounded, with the army now 35 kilometres (20 miles) away.

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 IS.

Earlier Tuesday, the national flag was raised throughout government-held areas of the city in anticipation of celebrations upon the arrival of government soldiers.

Some residents had begun greeting each other with the words: "Good morning of victory."

The army still faces a potentially difficult battle to break the siege on the south of the city and free its remaining neighbourhoods, and the surrounding province, from IS.

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.

- 'Spiral of defeats' -

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

IS has lost over half its other Syrian stronghold, the city of Raqa, to an offensive by the US-backed Syrian Democratic Forces, an alliance of Kurdish and Arab fighters.

And in neighbouring Iraq, it has lost 90 percent of the territory it once held, including the city of Mosul.

Inside Deir Ezzor, residents have faced years of privation, with food becoming scare or unaffordable, and medicine and healthcare unavailable.

The government has continued to fly in limited supplies by helicopter, and the UN last year began airdropping humanitarian aid to the city.

Syria's army began its offensive to reach the city in earnest last month, and has advanced on multiple fronts, including from the neighbouring Raqa province to the west and central Homs province to the south.

It has been supported by Russia's military, which began an intervention in support of the government in 2015.

More than 330,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests before spiralling into a vicious and complex civil war.

Tentative ceasefire zones have brought quiet to much of the country, but fighting rumbles on in pockets, including parts of rebel-held Eastern Ghouta outside the capital Damascus.

The Observatory said government shelling on the town of Mesraba in the region killed six people on Tuesday.