DAMASCUS, Syria -- Al Qaeda militants in northern Syria captured a town near the Turkish border Thursday following heavy clashes with Western-backed rebels, prompting the closure of a nearby frontier crossing, activists and Turkish officials said.
It was the latest development in what has effectively become a war within a war in northern and eastern parts of Syria -- pitting moderate fighters and Kurdish militiamen against extremists with ties to al Qaeda in recent battles that have left hundreds dead from both sides.
The U.S. and its European and Gulf allies are increasingly concerned about the rising prominence of Islamists among the rebels, who have played a major role in the civil war against President Bashar Assad's forces.
Elsewhere, a bus struck a roadside bomb in the central province of Homs, killing 19 people, a local official said. The blast in the village of Jbourin also wounded four people, according to the official from the governor's office who spoke on condition of anonymity because he was not authorized to talk to the media.
The village is predominantly Alawite -- an offshoot of Shiite Islam and a minority sect of which Assad is a member -- but it also has Christians and Sunni Muslims.
It was not immediately clear why the bus was targeted. The civil war, which has left more than 100,000 dead, has taken increasingly sectarian overtones. Most of the rebels trying to overthrow Assad belong to the majority Sunni sect.
The fighting in the north prompted Turkey to close the border crossing of Bab al-Salameh, a Turkish Foreign Ministry official said, speaking on condition of anonymity because he was not authorized to talk to the media.
The Britain-based Syrian Observatory for Human Rights, an activist group that monitors the conflict, said members of the al Qaeda offshoot, known as the Islamic State in Iraq and the Levant, stormed the town of Azaz on Wednesday evening, forcing opposition fighters from the Western-backed bloc to pull out.
Clashes between both sides broke out when ISIL 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 showed dozens of gunmen with heavy machine-guns on pickup trucks gathering at the border with Turkey. The Associated Press was able to verify the footage based on interviews and other reporting on the events depicted.
Abdul-Rahman said three opposition fighters and two jihadis were killed. On Thursday, mediation was underway to get the al Qaeda-linked militants to leave Azaz, he said.
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 jihadi fighters has been a worrying development, and FSA members have increasingly been on the offensive against the al Qaeda militants, whom they feel have discredited the rebellion.
The U.S. and its allies have been reluctant to get involved militarily in Syria or give lethal weapons to the rebels because of concerns the munitions would end up in the hands of jihadis.
The two sides have engaged in retaliatory killings in recent months. Kurdish militiamen have also been fighting against members of the ISIL 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.
According to Charles Lister, an analyst with HIS Jane's Terrorism and Insurgency Centre in Britain, al Qaeda-linked fighters make up between 10,000 and 12,000 of the insurgency's estimated 100,000-member force but wield far more influence because of their better discipline and battle experience.
Jihadis "represent a comparatively small minority of the total insurgent force, but as a result of superior finances, organizational capacity and individual fighter subservience to tight command and control structures, they have been able to exert far more of an impact on the conflict than some larger and more moderate insurgent forces," Lister said in a statement.
Also Thursday, Oxfam 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 UN's $5 billion appeals 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.
The fighting has forced 7 million people from their homes since the uprising began in March 2011. Five million Syrians have been displaced inside the country and more than 2 million have fled to neighbouring Jordan, Lebanon, Turkey and Iraq, the UN said.
