The column of eight rebel fighters picked their way along the street in Deir el-Zour at sunset, moving carefully through the debris of crushed glass and concrete, keeping their heads low and backs bent under the weight of the guns and rockets they carried.

They worked their way along the cratered road, past buildings chipped with multiple bullet holes and apartments and shops that had spilled out their contents on to the warm tarmac: burned mattresses, sofas, a fridge.

Nearby, mortars and shells were pounding out a rhythm. The men stopped in front of a collapsed building whose remaining walls were black with soot. There was a stench of rotten bodies. "We lost three men here two days ago," said the commander. He pointed at three dark puddles of congealed blood. "They lay here next to each other."

One fighter picked up a melted black flipflop. Another picked out the charred remains of a man's robe. He sniffed it. "This belonged to Abu Qutada," he said. "This is the smell of a martyr." He tucked the stinking fabric into his bag.

The eight men moved off, darting across the burned and newly named Freedom Square in the centre of the eastern Syrian city, zigzagging through the arcades that once housed shops selling gold, spices and electrical goods, but now home to packs of dogs half-crazed by the shelling. In Deir el-Zour, the battle appears to be on an endless loop. Every day, loyalist troops and tanks stubbornly try to take the city from the rebels.

The rebels push them back and the army retaliates by pounding the city with mortar shells and rockets.

The barrage starts in the morning and stops at midnight. Its aim is arbitrary: shells can land almost anywhere in the city.

The men split into three teams. A sniper climbed into a tall building overlooking a no man's land shared by government and rebel snipers. The rest surrounded a government-held checkpoint. The RPG launcher fired first, releasing a thundering boom, a huge cloud of dust and the sounds of cascading glass, metal and masonry. Silence. The government soldiers were not drawn into the firefight.

A chubby rebel ran into the middle of the street, firing a volley of bullets. The loyalist soldiers returned fire with a heavy machine gun, the buildings and street around the fighter exploding with yellowish sparks. The rebels escaped into an alley. The burly rebel commander ordered his men to the other side of the street, opening fire at the distant soldiers as cover until all the men had crossed. They took cover in a vegetable market, among steel stalls piled with rotten potatoes and onions, the smell of rotten vegetables and dead animals mingling with the scent of gunpowder.

When darkness fell the rebels withdrew. The sniper joined them in a dark alleyway. He was cheerful. They had managed to kill five soldiers, he said.

One of the men leaned against a nearby wall and began to throw up.

The siege

Until recently, when more sophisticated weapons began to flow in from Turkey, the province of Deir el-Zour was the main supply route for the rebel arms and ammunition which came over the border from neighbouring Iraq. Now most of the desolate countryside in the region is in the hands of the rebels, including the main border post.

"We control 90% of the province," said Abu Omar, a defected Syrian army major with a thick beard who headed of one of the two military councils leading the fight. "Is this province liberated? Not yet. We have more men, but they have the bases and we can't capture them without ammunition."

According to the rebels, a month of fierce fighting and artillery bombardment in Deir el-Zour city has seen hundreds of civilians, rebel fighters and loyalist soldiers killed, and 86 tanks and armoured vehicles destroyed.

But even as the civil war has moved into Damascus, the regime's security forces have continued to fight on this far edge of the country. In the past week government forces managed to take over two major intersections in the city, occupying them with tanks and establishing sniper positions. Many of the rebels are close to exhaustion. Food is served once a day to the fighters and supplies have dwindled to a trickle. They take four hours to travel a gruelling route through government lines.

The soldiers fare better than the civilians, however, as smuggled food comes with smuggled ammunition. The civilians are reduced to begging food from the fighters. One day during a week-long stay, a woman approached us.

"We need food. I have four kids and nothing to feed them," she said. "I will send you some tins later," said the fighter, sounding tired. "I have asked three units before and no one gave me anything," the woman retorted, before walking away.

The ragtag army can fight a war of attrition with the government, but with no leadership and no command structure, they are unable to organise a concentrated attack on its bases.

Opposition forces in Deir el-Zour are organised into around 20 battalions. The fighters consist of secularists and salafis, townspeople and tribesmen from the country, civilians and defected soldiers. They frequently bicker among themselves and accuse each other of hoarding weapons.

Some units have lost 70% of their men through casualties and desertion, and ammunition in some cases is so low that soldiers go to battle with one magazine. Others, however, hold stockpiles of brand new RPGs, Austrian-made machine guns and hand grenades, part of a shipment that the fighters say was bought with private money from Syrian donors and delivered by Turkish military intelligence over the border.

There are more weapons and men in the countryside, but many commanders prefer to protect their villages than send their men and weapons to fight in the city.

Meanwhile, the civilians who still make up most of the fighting force and who have carried the burden of fighting for the past 16 months look at officers who have defected recently with suspicion and resentment.

Khalil al-Burdany is a former English teacher who leads one of the main battalions in the town. The morning the Guardian met him, a column of pro-Assad tanks and soldiers had tried to get into the sector held by his battalion in the Umal area to the south of the city. A hundred rebels were scrambled and moved towards the front to help Khalil's small unit, but when government soldiers started firing mortars and tank rounds, half the men retreated. Only 15 of the reinforcements reached the front, where they stood behind a corner for an hour awaiting orders and then withdrew.

Khalil said: "Some of the battalions are just sitting eating and drinking and others are fighting. I had 50 men in this sector, now I have 23. The rest are dead.

"For 30 days I fought and I lost men every day." Khalil pointed to a burly major sitting in front of him who had defected a week earlier and continued in English: "This officer, he comes now and wants to become the supreme commander. They still have the Bashar [Assad] mentality and they only defected because they realised that we are winning."

After failing to get more men for his unit from the major, Khalil stormed out of the meeting to join his men. He was followed by a short teenage fighter. As they approached the front, they crouched behind a corner and inspected the road ahead.

A tank was stationed there, firing a shell every five minutes. They waited for one round to pass – it was followed by a thundering explosion – then Khalil ran across the street carrying his Kalashnikov in one arm. A burst of machine-gun fire kicked up the dust in the middle of the road.

The two men crossed two more streets with bullets whizzing around them before they met up with the rest of their men, a dozen soldiers who were taking cover from a regime tank. The rebels moved through holes dug between walls and took pot shots at government soldiers while the tank, unable to manoeuvre in the narrow road, fired shell after shell apparently aimlessly, sometimes striking close enough to the fighters to shower them with glass and plaster. Mostly the rounds landed far away.

In the middle of the road in front of the tank lay the smouldering wreckage of a pickup truck. It had been trying to evacuate two of Khalil's fighters when it was hit. One man lay in the middle of the road, half of his head blown away, his body swelling in the sun. The other, a charred, legless corpse, was still in the truck.

Khalil crawled into the middle of the road on his stomach to try to reach the bodies, but the car was too near the tank.

He made his way back and we headed for a house on the edge of the road. In the house's courtyard, a fighter stood on a stool and started picking white grapes from a vine.

"If we lose this area, we will lose Deir el-Zour," he said. "How would all the weapons hoarded in the countryside be useful then?"

On the other side of the road five of Khalil's men sneaked behind a group of government soldiers and opened fire. The soldiers retreated into a house carrying two injured men. The five men surrounded the house but couldn't get close because of sniper fire from a government position.

The tank fired round after round, trying to help the besieged soldiers, but it didn't move forward, fearing the IEDs planted by Khalil and his men. The rebels waited until nightfall, then surrounded the house and torched it with the soldiers inside.