Turkey closed a border crossing to Syriaafter an al Qaeda-linked group stormed a nearby town and expelled opposition fighters from an Arab and Western-backed unit, officials said on Thursday.

Fighters from the Islamic State of Iraq and the Levant (ISIL) on Wednesday killed at least five members of the Northern Storm Brigade, a rebel unit that controls the border, highlighting the deep opposition divisions.

The confrontation in the town of Azaz was one of the most serious clashes between the al Qaeda affiliate, made up mostly of foreign fighters, and the more ideologically moderate home-grown rebels trying to topple President Bashar al-Assad.

Their struggle, however, is less about ideology and more about a fight for territory, resources and the spoils of war - with armed ISIL fighters positioned to defend the town and a nearby rebel brigade trying to broker a ceasefire.

A Turkish official told Reuters the Oncupinar border gate - about 5 km (3 miles) from Azaz and opposite the Syrian Bab al-Salameh gate - had been closed for "security reasons".

"There is still confusion about what is happening on the Syrian side. All humanitarian assistance that normally goes through the gate has ceased," said the official, speaking on condition of anonymity.

Crossings such as Azaz have been a lifeline for rebel-held territories in Syria's north, allowing in humanitarian aid, building materials and food as well as giving refugees a route out of Syria.

While Turkey says it normally operates an open door policy, from time to time it temporarily closes its border crossings following clashes near the frontier.

The crossing fell into opposition hands last year when rebels launched an offensive to take the northern business hub of Aleppo.

Ankara has been one of the strongest backers of the rebels in the 2-1/2-year uprising against Assad. While it denies arming them, fighters including militant Islamists have been able to cross its border into Syria.

At the same time, many activists and Kurdish forces accuse Turkey of allowing radical groups to go through its territory to launch attacks on its other foe - Kurdish militias, who are now operating on the frontier in northeastern Syria. Turkey denies those charges.

REBEL DIVISIONS

Syrian activists said the fighting in Azaz had subsided by Thursday and there were no rebel preparations under way to take the town back from ISIL by force.

ISIL fighters were now spread throughout Azaz and had positioned snipers on rooftops, activists said.

Northern Storm fighters were stationed at the border crossing, where they were joined by fighters from the powerful Tawheed Brigade who came from Aleppo to try to broker a truce. Tawheed has a large presence in Aleppo, Syria's largest city, about 30 km south of Azaz.

"Reinforcements from the Tawheed Brigade were sent to impose a ceasefire on the two sides," said Abu Obeida, a Tawheed spokesman. "There is still no ceasefire yet ... There are negotiations under way."

The clashes were a stark illustration of the relative strength of the al Qaeda-linked fighters compared to Syria's larger but less experienced moderate forces. It also highlights the divisions that have plagued the opposition.

Both dilemmas have left Western powers hesitant to supply the rebels with advanced weapons.

ISIL declared an offensive last week against two other rebel factions, accusing them of attacking its forces and suggesting they may have collaborated with the government.

"What is worrying are the clashes themselves," a second Turkish official said, referring to rebel infighting generally.

"What we want is to see the various coalition groups put their house in order and focus on the struggle with the regime, because that is the real issue - the violence inflicted by the regime on the Syrian people."

An activist from Azaz who identified himself as Mohamed al-Azizi said he expected more violence before the confrontation was over.

"These people are very dangerous for Syria," he said via Skype, referring to the ISIL fighters. "They say they're Islamists but they have nothing to do with religion."