ANKARA/BEIRUT - Turkey closed a border crossing to Syria after 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.