* New rebel front formed against Islamic State fighters
* Dozens killed in rebel infighting since Friday
* Opposition appeals for support for rebel Free Syrian Army
By Dominic Evans
BEIRUT, Jan 4 (Reuters) - Syrian rebel factions battled fighters from the Islamic State in Iraq and the Levant (ISIL) across north-west Syria on Saturday in the heaviest clashes between President Bashar al-Assad's opponents in nearly three years of conflict, activists said.
The apparently coordinated strikes against the ISIL come after months of increasing resentment of the powerful al Qaeda-linked group, whose radical foreign jihadis and have alienated many ordinary Syrians in rebel-held territory.
Activists said dozens of fighters were killed in the clashes between rival rebel groups which have raged since Friday in Aleppo and Idlib provinces, close to the border with Turkey.
Rebel infighting has strengthened Assad's hand ahead of planned peace talks in Geneva on Jan. 22. The president, backed by Shi'ite fighters from Iraq and Lebanon's Hezbollah militia, has pushed back rebels around Damascus and in central Syria, and faces little pressure to make concessions.
One group of fighters battling ISIL was the newly formed Mujahideen Army, an alliance of eight brigades who accused the al Qaeda affiliate of hijacking their struggle to topple Assad.
They said ISIL fighters were "undermining stability and security in liberated areas" through theft, kidnapping and trying to impose their own brand of Islam, and vowed to fight them until ISIL was disbanded or driven out of Syria.
In response, ISIL pledged to fight back. "The blood of our brothers will not be shed in vain," it said in a statement.
Fighters from the Islamic Front, made up of several Islamist brigades which in the past were close to ISIL, were engaged in heavy clashes with the group in northern Aleppo province, according to the Syrian Observatory for Human Rights.
The Observatory, a monitoring group based in Britain, said at least 60 people had been killed in fighting which it described as a major challenge to ISIL's authority.
The Observatory and other activists also cited unconfirmed reports that Islamic State fighters in the village of Harem, about two km (one mile) from the Turkish border, killed 30 prisoners after they were surrounded by rebel fighters.
The ISIL and another al Qaeda affiliate, the Nusra Front, together with Islamist fighters from the Islamic Front, have eclipsed the Free Syrian Army which Western powers had hoped would grow into a moderate force capable of overthrowing Assad.
That impotence was highlighted in November when the FSA's military command lost control of a military base and main weapons depot near the Turkish border to the Islamic Front.
BATTLING QAEDA 'OPPRESSION'
Assad's main political opponents in exile, the National Coalition, sought to portray Saturday's clashes as a counter assault by the FSA against ISIL's "authoritarian oppression"
"The Syrian people clearly have rejected al Qaeda's attempts to establish a presence in the liberated territories," coalition member Monzer Akbik said. "The solution to fighting extremism in Syria is to strengthen the Free Syrian Army at this critical juncture".
The coalition said the fighting erupted after ISIL gunmen fired into a crowd of civilians in the northern village of Kafr Takharim who were commemorating the death in ISIL custody of a prominent Syrian doctor and rebel commander, Hussein Suleiman.
Suleiman's body was handed over by ISIL on Tuesday as part of a prisoner swap between rival rebel forces. Video footage of his corpse showed signs of beating and one ear was cut off.
Several demonstrations were held across Aleppo to mark Suleiman's death on Friday. Some brought together several hundred protesters, a dim echo of the many thousands who took to the streets for anti-Assad protests in the early months of the uprising, before it turned into armed insurgency and civil war.
More than 100,000 people have been killed in nearly three years of conflict. Two million refugees have fled abroad and another 6.5 million are internally displaced within the country of 23 million, the United Nations says.
The war pits Sunni rebels against forces loyal to Assad, from the Alawite faith which is an offshoot of Shi'ite Islam, and has divided the Middle East along sectarian lines, with Sunni states such as Turkey and the Gulf monarchies backing the rebels, and Shi'ite Iran and Hezbollah supporting Assad.
Western reluctance to intervene militarily in the conflict - in contrast to the rapid NATO involvement in Libya in 2011 - has been heightened by concerns about the growth of al Qaeda-linked Sunni Muslim groups in rebel areas of north and eastern Syria.
Their spread inside Syria has been matched across the border in western Iraq, where ISIL has tightened its grip in the Sunni Muslim province of Anbar.
The group also claimed responsibility for a suicide bombing in southern Beirut which killed five people on Thursday, saying it was the start of a campaign targeting Hezbollah. (Editing by Rosalind Russell)