A Free Syrian Army fighter carries his weapon as he peeks out from a damaged shop in the old city of Aleppo Nov. 29, 2013. Reuters AMMAN (Reuters) — Armed with machine guns, black-clad al Qaeda fighters drove their pick-ups calmly into the northern Syrian town and took over its imposing agriculture ministry building.

They beheaded a sniper from a rival rebel unit, displayed his head in the main square and put roadblocks on major routes.

Not a shot was fired in the takeover, in which informants, including a preacher from a local mosque, played key roles.

The scene in Termanin, recounted by an activist who witnessed it last week, is being repeated in towns along the border with Turkey and at road junctions further inside Syria that have fallen out of President Bashar al-Assad's control.

Whether through weakness or a desire to focus on Assad, rebel units are making way for the Islamic State in Iraq and the Levant (ISIL), an al Qaeda affiliate led by foreigners hardened by guerrilla warfare in Iraq, Chechnya and Libya.

The land grab has given radical jihadists a large territorial base in the heart of a Middle East convulsed by the civil war raging in Syria since 2011.

While constant conflict and shifting alliances mean Syria is a long way from becoming a center for global jihad, Western and Arab states backing moderate opponents of Assad are alarmed.

The ISIL is taking over supply lines to rebel areas and attracting members of less organized opposition units by its efficiency, undermining efforts by Washington to contain it ahead of talks in Geneva on a possible peace deal, opposition sources and Middle East security officials say.

As well as an end to Assad's rule, a key aim of such a deal would be to establish a government and moderate army capable of fighting off the ISIL, a Middle East-based diplomat said.

"Realistically it will be very difficult. We could be looking at a proxy sectarian war — whether Assad stays or goes — in which the ISIL will be a major player."

LESSONS FROM LIBYA

Asked about the group's goals, an ISIL commander in the town of Armanaz in northern Syria who had fought in Libya said it is fighting for "the downfall of the tyrant Bashar" but also seeking to impose Islamic law.

Learning lessons from the 2011 war in Libya, he said ISIL was more determined to hold on to territory under its control.

"Our mistake as mujahideen is that we were preoccupied with fighting Gaddafi and did not pay enough attention to how to hold on territory," said the commander, who goes by the nickname al-Jazaeri, or the Algerian.

In a sign of concern over ISIL's gains, the United Arab Emirates, a staunch U.S. ally, convened a meeting last week for dozens of tribal leaders from the oil-producing region of east Syria bordering Iraq's Sunni heartland.

The ISIL and the Nusra Front, a smaller affiliate of al Qaeda seen as less intent on spreading jihadist ideology, occupy most oil fields in the eastern province of Deir al-Zor, although they lack the ability to operate the wells.

The UAE meeting aimed to gauge the possibility of setting up a force similar to the Sahwa movement that fought al Qaeda in Iraq and rolled back some of its influence, opposition sources said, although neither the tribes nor the Islamists appear ready for a sustained fight.

"There have been some clashes over oil but the ISIL has sought not to mess with the tribes. At the same time the tribes are seeing how the ISIL likes to chop heads and they too are not keen on a confrontation," one of the sources said.

GROWING POWER

Areas under ISIL control include towns across the northern Syrian provinces of Idlib and Aleppo, parts of the eastern provincial capital of Raqqa and, to a lesser extent, of central Syria and the southern province of Deraa.

In some of these it is trying to implement a rigid Islamist social agenda and has also won new recruits, attracted more by its effectiveness than its ideology, local activists say.

In the al-Rouge plain in Idlib, bordering Tukey, Hassan Abdelqader said ISIL has set up training camps for local recruits and has distributed head to toe veils in areas southeast of Idlib city to be worn by women there.

In al-Bab in Aleppo province, where Abu Mouawiya, an ISIL commander, is the effective governor, the group has enforced an Islamist school curriculum imported from areas under al Qaeda control in Yemen, activists said.

They said thousands of poor Sunnis from rural Idlib and rural Aleppo have joined the ISIL in the last few months, including fighters who had left Nusra and the Free Syrian Army — the Western-backed force that aims to unite moderate rebels.

A Free Syrian Army report prepared for the U.S. State Department and quoted by the Washington Post said the ISIL has a backbone of 5,500 foreign fighters, including 250 Chechens in Aleppo, and 17,000 recruited locally.

The local recruits are rural Sunnis, the majority group at the forefront of the uprising which grew from a crackdown on protests against four decades of rule by the Assad family.

The Assads are from the Alawite offshoot of Shi'ite Islam backed by Iran and Hezbollah, while the Sunni rebels are supported by Gulf heavyweights and Turkey, but inter-rebel clashes have blurred the conflict.

In some areas ISIL works with rivals from Nusra and the Western backed Free Syrian Army while in others it fights them. A new alliance comprising big Islamist brigades also has a mixed relationship with ISIL.

FLUID ALLIANCES

In southern Damascus, ISIL has joined Nusra and other brigades in defending opposition neighborhoods from advances by Assad's forces backed by Iraqi and Lebanese Shi'ite militia.

Just months before, ISIL had attacked Nusra positions, taking advantage of an air strike by Assad's forces that killed three Nusra commanders, local rebel sources said.

Nusra, ISIL and Free Syrian Army units also cooperate in the northeast, where they are fighting what they consider a land grab by Kurdish PYD militia. The PYD says it is defending the population against al Qaeda.

In areas along the border with Turkey in Aleppo and Idlib, where the presence of Assad's forces is limited, ISIL has been more assertive in taking over territory from the moderate Free Syrian Army and other hardline Islamist units.

Activist Firas Ahmad, who witnessed the takeover of Termanin, said it was typical.

"They have informants who identify a weak target in a town. They also capture the bakery and put roadblocks at the main roads, ensuring that they control food and movement."

This brings ISIL revenue as well as supplies destined for the other brigades.

"The executions are designed to make maximum impact," said Ahmad, pointing to amateur video showing the ISIL executing the leader and several members of Ghurabaa al-Sham, a moderate Free Syrian Army unit, in Atarib.

Members of a rebel-run police station near Hazano town were spared a similar fate after the station was stormed by ISIL, Ahmad said. "The police chief and staff surrendered as soon as the attack started and declared their allegiance to the ISIL."

Last month the ISIL took three pick-up trucks equipped with anti-aircraft guns that had crossed through the town of Atma, and came close to taking other trucks carrying thousands of U.S. supplied combat food rations, activists said.

An opposition figure who attended a meeting with U.S. officials about logistics said: "The Americans are furious at the degree of ISIL reach over supply lines. Privately they are blaming the Turks for opening their borders in such a way that facilitated the infiltration of al Qaeda."

Turkey has been an outspoken supporter of rebels fighting Assad and has assisted them by keeping its border open, but Turkish Prime Minister Tayyip Erdogan and other officials have strongly denied this amounts to support for his al Qaeda foes.

Abdallah al-Sheikh, an activist in Atma, said Turkish soldiers have been intercepting supply conveys and seizing them or turning them back in the wake of the recent ISIL advances.

"The end result is that the ISIL is harming the overall military struggle and doing Assad a service," Sheikh said, adding that signs of a popular backlash against ISIL and the group's interference in people's lives are beginning to emerge.

In the last few days, Sheikh said, the ISIL had been forced to withdraw from the nearby village of al Qah after armed skirmishes with local residents.

(Editing by Dominic Evans and Philippa Fletcher)