BEIRUT (AP) -- Syrian rebels surrounded a compound held by al-Qaida-linked fighters and freed at least 50 people from a nearby prison Monday as clashes between rival factions in the country's northern provinces spread to the largest city controlled by the opposition.
The rebel-on-rebel fighting in Raqqa -- a stronghold of the al-Qaida-linked group known as the Islamic State in Iraq and the Levant -- reflects a widening war within a war in Syria, this one against radical extremists. It also adds yet another layer of complexity to the broader Syrian conflict less than three weeks ahead of a planned international peace conference to try to broker a political solution to the civil war.
Support from the U.S. and its Western allies for the rebels has faded in the past year as al-Qaida-affiliated groups have risen to become one of the most dominant forces among the patchwork of opposition fighting factions.
There was no indication that the move by a mix of more moderate rebels and powerful ultraconservative Islamist brigades against the al-Qaida fighters was a reaction to Western pressure to move against the extremist group. Rather, the violence has been largely limited to communities where tensions between the factions were already simmering.
The number of towns, villages and neighborhoods where clashes were taking place spread across four provinces, providing an indication of the extent of resentment of the Islamic State in Iraq and the Levant.
Since spring 2013, the group has muscled its way into rebel-held territory across northern Syria, crushing resistance from other factions, seizing their weapons and detaining their fighters. It has kidnapped journalists and abducted activists who are critical of its efforts to impose a strict interpretation of Islam.
For months, sporadic clashes between its fighters and other rebel brigades have left scores dead and hampered the broader movement to topple Syrian President Bashar Assad.
But the latest fighting, which broke out Friday in the northern provinces of Aleppo and Idlib after residents there accused the al-Qaida-linked group of killing a popular doctor, is the most serious since the uprising began in March 2011.
The fighting has since spread to the central province of Hama as well as the northeastern province of Raqqa, and killed an estimated 100 fighters on both sides, according to the Britain-based Syrian Observatory for Human Rights activist group.
The fighting in the city of Raqqa -- the provincial capital -- began before dawn Monday, when a coalition of Islamic brigades attacked fighters from the Islamic State in Iraq and the Levant, said Observatory director Rami Abdurrahman. Another activist group, the Local Coordinating Committees, also reported the Raqqa clashes, saying they were focused around a post office.
The Observatory said rebels surrounded the group's main compound in Raqqa and freed at least 50 detainees from a prison. Abdurrahman said they included fighters and activists imprisoned for criticizing the group.
"Many of them were from various rebel groups," Abdurrahman said. "Some of them had been held a few weeks, some of them a few months."
The Observatory also said the Islamic State in Iraq and the Levant retreated from the town of Tal Abyad along the Turkish border after heavy fighting there. Clashes continued in parts of Aleppo province, including the neighborhoods of Masaken Hanano and Shaar in the city of Aleppo.
Raqqa was the first provincial capital to fall entirely into the hands of rebels, and the Islamic State in Iraq and the Levant slowly squeezed many other groups out of the city last year. An Italian Jesuit priest, the Rev. Paolo Dall'Oglio, disappeared in July after traveling to meet al-Qaida militants in Raqqa. Since then, rumors have swirled that foreign aid workers, reporters and Syrian activists have been held in Raqqa's detention centers.
Abdurrahman estimated that more than 1,000 people are being held in Raqqa province. He said the fate of the priest and foreign journalists was unknown.
Activists said another al-Qaida-linked rebel group, the Nusra Front, was taking part in the fighting in Raqqa against the Islamic State in Iraq and the Levant. However, Abdurrahman said many of the Nusra Front fighters in Raqqa had joined the group from other rebel outfits that had collapsed in the face of the Islamic State in Iraq and the Levant.
One of the more moderate rebel groups involved in the fighting elsewhere against the al-Qaida-linked insurgents, the Syrian Revolutionary Front, appeared reluctant to give up the offensive.
The group, a recently formed coalition of rebel brigades aligned with the exiled Syrian opposition, demanded in a statement Monday that the al-Qaida-linked fighters desert and join their ranks. They also blamed the group for killing at least 400 of their fighters and imprisoning 2,000.
"Were it not for these actions, the Front would not have raised its weapons," the statement said.
Rebels appear to have taken several of the group's compounds in Aleppo province, and its fighters handed over strongholds in the town of Tal Rafat to the Nusra Front to avoid clashes, the Observatory reported.
At the United Nations, Secretary-General Ban Ki-moon was sending out 20 invitations to the so-called Geneva peace conference, which is to open Jan. 22 in Switzerland. Iran, a close Assad ally that has supplied his government with weapons, funds and military advisers, was not among the nations invited, U.N. spokesman Farhan Haq said.
The U.S. and Russia have not yet agreed on Iran's role in the talks, but Haq says the U.N. hopes that can be resolved at a Jan. 13 meeting between U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov.
Associated Press writers Zeina Karam and Diaa Hadid in Beirut and Peter James Spielmann at the United Nations contributed to this report.