Soldiers metres away from IS in Syria's Deir Ezzor

A Syrian soldier shoots through sandbags at a black flag of the Islamic State group just several metres away in the divided eastern city of Deir Ezzor.

Besieged troops in the city's government-held districts are on alert day and night for IS attackers after more than two years of siege by the jihadists.

"We never sleep. Even at night, we keep an eye open," says Omar, asking for a pseudonym like all other soldiers in the city interviewed by AFP.

President Bashar al-Assad's army controls around 40 percent of Deir Ezzor, the only city in the whole country where his troops are besieged.

"Even on our breaks, we stay vigilant," Omar says, as he hunkers down behind sandbags several metres (yards) away from jihadist positions in the front line neighbourhood of Huwayqa.

A 10-metre-long trench padded out by sandbags allows the soldiers to run along the front line and spy on their adversaries in the divided district, whose buildings have been devastated in the fighting.

The bearded soldier, who carries a Kalashnikov assault rifle and ample ammunition, turns to congratulate a fellow fighter for having hit an IS standard marking one of the jihadists' positions.

Omar is on guard in the only eastern city to have a regime-held district in a part of Syria largely controlled by IS.

He has not seen his family in Damascus for three years.

Nearby, an officer says the sandbags provide protection from snipers, but also allow the military to "dig tunnels to reach enemy positions without being seen".

Only helicopters can reach Deir Ezzor's regime-held western districts after IS took over part of the city in July 2014 and laid seige to government-controlled areas in January 2015.

For Assad's army, staying in the city means one day returning to the Euphrates valley and retaking oil fields that account for two-thirds of the country's production, says Fabrice Balanche, a Syria expert at the Washington Institute for Near East Policy.

And Deir Ezzor is the second most important city that IS controls after its de facto Syrian capital of Raqa, which is in the sights of an offensive by a US-backed Arab-Kurdish alliance.

- Attacks in low visibility -

The surrounding province of Deir Ezzor -- the second largest in the country -- is an important area for agriculture, Balanche says.

The army controls the provincial capital's west where around a third of the city's pre-war population of 300,000 live, as well as the military airport on its outskirts.

IS holds the city's east, where about 50,000 civilians live.

"Our positions are just 15 metres away from the enemy," the leading officer in Huwayqa says, asking to remain anonymous.

He says the jihadists do not lead frontal attacks but try to infiltrate army ranks -- especially in low visibility.

"They take advantage of sand storms, fog or frost to attack. When these happen, we become even more vigilant," he says.

The military does not release death tolls, but a monitor says 2,500 soldiers and pro-regime fighters have died in Deir Ezzor since the battle started there.

The Syrian Obervatory for Human Rights monitoring group estimates around 3,000 jihadists have also died.

Both sides have entrenched their positions in the past two months, with no advance from either after months of fighting for now seasoned soldiers.

"I fought rebels, the jihadists from Al-Nusra Front, and then IS," says Samer, referring to the group that renamed itself Fateh al-Sham Front after renouncing its ties with Al-Qaeda.

"Rebels lead frontal attacks and Al-Nusra puts mines in buildings to blow them up, while IS sends us car bombs followed by suicide bombers," Samer says.

In Deir Ezzor, the army has been fighting one of its toughest battles against IS, a group that has become infamous for gruesome beheadings.

But Samer seems to have kept his morale.

"They want us to believe they'll chop our heads off, but we'll hold on tight."