* Islamist alliance had announced withdrawal from FSA
* Attack a blow to Western efforts to unite rebels
* Two al Qaeda-linked groups reportedly fight in Raqqa
By Alexander Dziadosz and Dasha Afanasieva
BEIRUT/ISTANBUL, Dec 7 (Reuters) - Syrian rebels from an Islamist alliance formed last month have occupied bases and warehouses belonging to a Western-backed rebel group on the Turkish border, rebels and activists said on Saturday.
Fighters from the Islamic Front, a union of six major rebel groups, took control of the Free Syrian Army (FSA) bases at the Bab al-Hawa crossing on the northwestern border with Turkey late on Friday night, the opposition sources said.
Louay Meqdad, an FSA spokesman, said the Islamic Front fighters had entered the bases after saying they wanted to help to secure them. They then asked officers and employees to leave and replaced an FSA flag with one of their own, he said.
"We believe that those brigades are our brothers, that they know that we are not the enemy," Meqdad said.
Infighting among Syria's rebel groups has undermined their fight against President Bashar al-Assad in the 2-1/2-year-old civil war and made Western governments hesitant to back them.
The rise of hardline Islamist groups among the rebels, including some linked to al Qaeda, has also unsettled powers such as the United States, who fear that if the militants came to power, they would eventually turn their weapons on Western targets.
On Tuesday the Islamic Front said it had withdrawn from the military command of the FSA, notionally charged with coordinating the war, and criticised its leadership.
While the Islamic Front does not include either of Syria's two al Qaeda-affiliated units - the Nusra Front and the Islamic State of Iraq and the Levant (ISIL) - it does include radical Islamists who have coordinated with them.
DIFFICULTY OF UNITING REBELS
On Friday, the opposition Syrian National Coalition published statements by an FSA official playing down the Islamic Front's withdrawal and denying that the groups were in disagreement.
But the events in Bab al-Hawa, confirmed by activists in the area, underscored the size of the task facing the Western powers as they try to unite rebel groups under FSA command and sideline more hardline groups.
Observatory director Rami Abdulrahman said five rebel fighters had been killed in clashes in Bab al-Hawa, but it was not clear which side they were on.
It is difficult for Reuters to independently verify reports from inside Syria because of media restrictions.
The Observatory, which has a network of sources across Syria, said fighting also broke out between ISIL and the Nusra Front on Friday in the northeastern city of Raqqa, the largest city to fall under rebel control so far.
The fighting started after a unit loyal to the Nusra Front killed a Saudi Arabian ISIL fighter who did not stop at one of the unit's checkpoints, the Observatory said, quoting its sources in the area. There were no reports of casualties.
Separately, an air raid killed at least 20 people, including eight children and nine women, in parts of Bezaa in the northern Aleppo province, the Observatory said.
Syria's conflict has killed more than 100,000 people since it started in 2011 as a peaceful uprising against four decades of Assad family rule.
The weekly death toll often climbs above 1,000, and world powers remain deadlocked over how to resolve the crisis. (Editing by Kevin Liffey)