The capture of a town in northern Syria on Thursday by a rebel group linked to al-Qaeda, and the subsequent closure of a nearby border crossing with Turkey, has underscored the increasing internal splits within the Syrian opposition as the two-year-old Syrian civil war grinds on.
The development comes after recent fighting between the al-Qaeda-linked Islamic State of Iraq and the Levant (ISIL) front and a mixture of fighters affiliated with, or loyal to, the Western-backed Free Syrian Army (FSA) has left hundreds dead on both sides.
Elsewhere Thursday, a bus struck a roadside bomb in the central province of Homs, killing 19 people, a local official said. It was not immediately clear why the bus was targeted, but the civil war, which has left more than 100,000 dead, has becoming increasingly sectarian in the ethnically and religiously varied society.
The fighting in the north prompted Turkey to close the border crossing of Bab al-Salameh, a Turkish Foreign Ministry official said.
The Britain-based Syrian Observatory for Human Rights, an activist group that opposes the government of Bashar al-Assad and who monitors the conflict, said members of ISIL first stormed the town of Azaz Wednesday evening, forcing opposition fighters from the Western-backed bloc to pull out.
Clashes between both sides broke out when ISIS fighters tried to detain a German doctor they accused of taking photos of their positions on behalf of the rival rebels, said Rami Abdul-Rahman, who heads the Observatory. The doctor, a volunteer in the region, escaped, but the two rebel factions started fighting.
Amateur video verified by The Associated Press showed dozens of gunmen with heavy machine guns on pickup trucks gathering at the border with Turkey.
Loay al-Mikdad, a spokesman for the Western-backed rebels of the Free Syrian Army, said the al-Qaeda-linked fighters "want to occupy the area."
"What they are doing is unjustified, it serves the (Assad) regime," al-Mikdad said by telephone from Turkey.
In July, ISIL fighters killed two FSA commanders. The deaths enraged the FSA leadership, which has demanded the killers be handed over for trial.
There has also been infighting among rebel groups in the eastern province of Deir el-Zour, which borders Iraq, and in the north where al-Qaeda fighters from ISIL and their allies in the Nusra Front have been battling Kurdish rebels for months.
The rise of extremist fighters in Syria has been a worrying development for U.S. policymakers and those in the Obama administration, where the participation of al-Qaeda-linked groups in the conflict against Assad has been given as a reason to oppose increased U.S. military support or intervention to the rebels.
Meanwhile, FSA members have increasingly been on the offensive against the al-Qaeda militants, whom they feel have discredited the rebellion.
The two sides have engaged in retaliatory killings in recent months. Kurdish militiamen have also been fighting against members of the ISIS and the Nusra Front in predominantly oil-rich Kurdish areas of northeastern Syria.
Residents of rebel-held areas are also turning against extremists for their brutal tactics and for trying to impose Islamic law.
The fighting in Syria has forced at least 6 million people from their homes since the uprising began in March 2011. At least 4 million Syrians have been displaced inside the country and more than 2 million have fled to neighboring Jordan, Lebanon, Turkey and Iraq, the U.N. said.
On Thursday, Oxfam, an international humanitarian agency, said many donor countries are failing to provide their share of the funding for the humanitarian response to the Syria crisis. Donors, including France, Qatar, United Arab Emirates and Russia, should make funding the U.N.'s $5 billion request a priority, the international aid agency said.
Oxfam's report came ahead of next week's donors' meeting in New York. The donors have been influential in shaping the international response to the conflict, but should also pay their fair share of the humanitarian aid, the agency said.
Al Jazeera and wire services