Syrian opposition fighters are seen at the top of a mountain in the vicinity of a rebel camp in Idlib province, Syria, Wednesday, Sept. The main Syrian opposition coalition urged the international community to take swift action against the regime of President Bashar Assad in response to a U.N. finding that the nerve agent sarin was used in a deadly attack near the capital last month. A Syrian opposition fighter smokes a cigarette as he sits at the top of a mountain in the vicinity of a rebel camp in the Idlib province countryside, Syria, Wednesday, Sept. A Syrian opposition fighter sits at the top of a mountain in the vicinity of a rebel camp in the Idlib province countryside, Syria, Wednesday, Sept. DAMASCUS, Syria (AP) - Al-Qaida-linked gunmen in northern Syria captured a town near the Turkish border after heavy clashes with a rebel group in the area, activists said Thursday. It was the latest development in what has been a relatively new component in the Syrian conflict - stepped-up infighting between extremists with ties to al-Qaida and Western-backed opposition groups. The U.S. and its European and Gulf allies are increasingly concerned about the rising prominence of Islamists among the rebels, who have been playing a major role in the battles against President Bashar Assad's forces. Elsewhere in Syria, a roadside bomb struck a bus in the central province of Homs Thursday, killing 19 people, a local government official said. The explosion in the village of Jbourin also wounded four people, according to the official from the governor's office who spoke on condition of anonymity in line with regulations. The village is predominantly Alawite, an offshoot of Shiite Islam and a minority sect to which Assad belongs, but it also has Christians and Sunni Muslims. It was not immediately clear why the bus was targeted but Syria's civil war, which has left more than 100,000 dead since the crisis erupted in March 2011, 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 nearby 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-Qaida offshoot known as the Islamic State in Iraq and the Levant stormed the town of Azaz in the northern Aleppo province on Wednesday evening, forcing opposition fighters from the Western backed bloc to pull out. 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-Qaida fighters from the ISIL and their allies in the Nusra Front have been battling Kurdish anti-government rebels for months. The infighting has left hundreds dead. The Azaz clashes broke out earlier Wednesday, when ISIL fighters tried to detain a German doctor they accused of taking pictures of their positions on behalf of the rival rebels, said Rami Abdul-Rahman, who heads the Observatory. The doctor, who was a volunteer in the region, escaped but the two rebel factions started fighting. Amateur videos showed dozens of gunmen with heavy machine guns mounted on pickup trucks gathering at the nearby border crossing with Turkey. The videos appeared genuine and corresponded to other Associated Press reporting on the events depicted. Abdul-Rahman said three opposition fighters and two jihadis were killed in the fighting. On Thursday, mediation was under way to get the jihadis to leave Azaz, he said. Loay al-Mikdad, a spokesman for the Western-backed rebels of the Free Syrian Army, said the Azaz clashes were a "provocation" by the al-Qaida-linked fighters. They want to occupy the area ... What they are doing is unjustified, it serves the (Assad) regime," Mekdad said by telephone from Turkey. In July, ISIL fighters killed two FSA commanders. The deaths enraged the FSA leadership, which has since demanded that the killers be handed over to stand trial. The rise of jihadi fighters in Syria has been a worrying development. According to Charles Lister, an analyst with HIS Jane's Terrorism and Insurgency Centre in Britain, al-Qaida-linked fighters make up between 10,000 and 12,000 of the insurgency's estimated 100,000-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 wrote in a statement Wednesday. Also Thursday, the international aid agency Oxfam issued an appeal, saying many donor countries are failing to provide their share of the urgently-needed funding for the humanitarian response to Syria crisis. Oxfam said donors, including France, Qatar, United Arab Emirates and Russia, should prioritize funding the U.N.'s $5 billion appeals. Oxfam's report came ahead of next week's donors meeting in New York. The donor countries have been influential in shaping the international response to the conflict, but should also bear their fair share of the burden of humanitarian aid, the agency said. "Too many donor countries are not delivering the level of funds that is expected of them," said Colette Fearon, head of Oxfam's Syria program. While economic times are tough, we are facing the largest man-made humanitarian disaster in two decades and we have to seriously address it. "The scale of this crisis is unprecedented and some countries must start to show their concerns to the crisis in Syria by putting their hands in their pockets," Fearon said. The fighting in Syria has forced 7 million people to flee their homes. Five million Syrians have been displaced inside the country and more than 2 million have sought refuge in the neighboring countries of Jordan, Lebanon, Turkey and Iraq, according to the U.N. Associated Press writers Desmond Butler in Istanbul and Lori Hinnant in Paris contributed to this report.