The young rebel stepped out from his battered sedan looking warily at the throng of passersby as he picked his way through festering rubbish bags piled in front of a school.

He pushed against a wrought iron gate and disappeared into the expanse of the empty schoolyard, invisible in the coal-dark of another power-less night in Aleppo.

"I have a problem with al-Qaida," he said from the gloom. "Come with me, alone, and I'll tell you."

He gripped his short black beard anxiously and began to speak. "I am an engineer," he said. "I trained abroad and I came back for this revolution. My skill is in making machinery parts and now al-Qaida want me to make their weapons. They run everything here. They are very powerful."

The group he called al-Qaida is known locally as Jabhat al-Nusra. Before the siege of Aleppo started mid-July, the group was unknown in the city and had been only a fleeting presence in the rebellious countryside.

Now though, almost six months later, inspired by the Bin Laden world view of a global jihad to enforce a fundamental Islamic society, al-Nusra is very much competing for influence in the Syria that will take shape if and when the embattled regime falls.

Through a growing role on the battlefield and a rising reputation as an organisation that can get things done, al-Nusra has become a player in the power vacuum that has emerged from the civil war. It is also increasingly known as an enforcer, whose unbending demands are unsettling regular rebel units as well as the societies the group claims to protect.

The rebel, who gave his name as Hassan, worried that eavesdroppers might somehow pick up his whispered English even in this vast, deserted playground, decided the noisy street outside would be a better place to talk.

Generators rumbled their relentless beat across a street slickened with oil, mud, and blood dripping from slaughtered lambs. Meat from the carcasses was being cooked on roadside barbecues, the smoke from the small fires mixing with a freezing fog, which seemed to thicken with the breath of vendors and customers as they huddled in the bitter cold, illuminated by gas lamps and candlelight.

"They [al-Nusra] come to me all the time," said Hassan, who was once part of a Free Syrian Army unit in the east of the city. "I make excuses with them, telling them that the machine part is not right for what they want – anything to avoid doing what they want. They want me to make their bombs."

Over the past two months al-Nusra has felt emboldened enough to step from the shadows. It has opened shopfronts in most towns and villages, from Aleppo to the Turkish border, and has even set up a headquarters in plain sight in the centre of the city, alongside the base of a regular Free Syrian Army unit, Liwa al-Tawhid.

A simple black Islamic banner, the same one adopted by al-Qaida in Iraq, from where many of al-Nusra's members hail, hangs at each of the outposts.

In these hubs al-Nusra cadres receive residents who come to them to resolve disputes and seek aid. Men with notebooks sit inside chronicling complaints and sometimes giving out vouchers for food or fuel.

Throughout history Syrians have sought out patrons, usually tribal sheikhs or chieftains, for assistance in all manner of things, from mediation to marriage. In some cases al-Nusra is now filling these roles.

"Where is the Jabhat [al-Nusra] base?" an elderly man carrying a plastic bag of medication and a handwritten letter asked as he walked along the verge of a main road in Aleppo.

"What do you want from them?" another man said.

"They are good people," he replied. "They can fix problems. I'm very tired of all of this suffering."

The man shuffled down the road to a gate of large horizontal bars, where a smiling al-Nusra cadre, with a ginger beard and green fatigues, ushered him inside to a waiting room.

Other al-Nusra members tinkered with a satellite link for a flat screen TV on the wall. More came and went from the reception area. Many limped from recent wounds, others bore visible scars of war.

In the heady months that followed the storming of Aleppo – a push that was led by the rural poor from the nearby hinterland – al-Nusra took prominent and decisive positions at most battles.

Their prowess as fighters has been acknowledged by other rebel units, who in the early days began to defer to the better-armed and organised jihadists among them.

It was, for a while, a tale of dramatic gains; of revolutionary zeal and a fervour to get things finished, no matter the methods or consequences. Ideological and religious differences were set aside in the battle against a common enemy – the Assad regime.

"But then they [al-Nusra] began to reveal themselves," said a senior rebel commander in Aleppo. "The situation is now very clear. They don't want what we want."

Over the past six weeks a once co-operative arrangement between Aleppo's regular Free Syrian Army units and al-Nusra has become one of barely disguised distrust.

A week of interviews with rebel groups in north Syria has revealed a schism developing between the jihadists and residents, which some rebel leaders predict will eventually spark a confrontation between the jihadists and the conservative communities that agreed to host them.

Some already talk of an Iraq-style "awakening" – a time in late-2006 as when communities in the Sunni heartland cities of Fallujah and Ramadi turned on al-Qaida groups in their midst that had tried to impose sharia law and enforce their will through the gun barrel.

"We'll fight them on day two after Assad falls," a commander said. "Until then we will no longer work with them."

In recent weeks Liwa al-Tawhid and other militias who form part of the Free Syrian Army have started their own operations, without inviting al-Nusra along.

A raid on an infantry school north of Aleppo was one such occasion; so are attacks against Batallion 80 on the outskirts of the city's international airport and a military base to the east, known as Querres.

"They are not happy with us," the rebel leader said. "But they had been hoarding all their weapons anyway."

Another significant issue for rebel leaders is what to do with state assets that have fallen into the hands of the opposition.

"They see stealing things that used to belong to the government, like copper factories, or any factory, as no problem," said the rebel commander. "They are selling it to the Turks and using the money for themselves. This is wrong. This is money for the people."

On Monday al-Nusra units went to a state-owned water factory on the Euphrates river. They invited regular rebel units to go with them as they picked through parts inside the factory for selling to whoever wanted them.

One unit did join the jihadists. Others refused.

"These are Syrian assets for Syrian people," said a rebel commander who did not want to be named. "They see this as an open pasture for them to do as they please. Our job is to protect the state for life after Assad, not to destroy it."

Money is flowing to al-Nusra. Members acknowledge that they receive cash from benefactors in the Sunni Arab world. But their coffers are also being filled with a garage sale of state assets, largely conducted by al-Nusra leaders.

A rebel unit pulled up on a main road in eastern Aleppo just up the road from the al-Nusra base. Pigeons circled over the city's ancient citadel, which soared from a hilltop in the near distance.

Another rebel approached, this time to complain that young girls in his village had been pledged as brides to anyone who joined al-Nusra. "This is part of the employment benefits," he said.

For now, community leaders seem to be able to say no to al-Nusra suitors who come calling, but fear these rights might be whittled away if the group consolidates its influence.

North of Aleppo, in the small forsaken hamlet of Dabek, al-Nusra, fighters had recently paid a visit. Their goal had been to damage a grave that was, in their eyes, too pretentious for Islamic traditions, which specify that all graves, no matter who is buried in them, must be modest.

"They damaged some of the rocks around the grave and they did the same in Azaz [a nearby town]," said the local sheikh. "They are starting to impose themselves and their values."

While the group's footsoldiers are now visible in north Syria, its leaders keep a low profile. Privately, some members, veterans of al-Qaida in Iraq among them, say community outreach is essential to the group's role in Syria.

"There were mistakes made in Iraq. Killing people on camera, being so visibly connected to sectarian fighting. These things cannot be repeated," one member said.

But assassinations of some captured regime soldiers that were recorded on camera and uploaded to the internet show restraint is not being practised by all al-Nusra members.

"There is discipline for those who don't follow the leaders," the member said. "We need the community and they need us."

A rebel commander told a story, as a warning of the dangers al-Qaida represented to Syrian society. Late last year the leaders of some towns in the Aleppo hinterland and the rebel commanders who move between them received word of a visitor.

"He was a Tunisian," said the commander. "And he said he brought a message on behalf of Ayman al-Zawahiri [al-Qaida's leader]. He asked us to join him and said there would be benefits for us if we did. He asked me to pledge a bayaa [oath of allegiance] to al-Qaida. I said no. This is what we all must do. If we continue with them, the Syria of our dreams will instead haunt our children in their nightmares."