BEIRUT — Syrian rebels surrounded a compound held by Al Qaeda-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 Raqqa, the largest city controlled by the opposition.
The rebel-on-rebel fighting in Raqqa — a stronghold of the Al Qaeda-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 United States and its Western allies for the rebels has faded in the past year as groups affiliated with Al Qaeda have risen to become a dominant force among the patchwork of fighting factions.
There was no indication that the move by a mix of more moderate rebels and powerful ultraconservative Islamist brigades against the Al Qaeda 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 President Bashar Assad of Syria.
But the latest fighting, which broke out Friday in the northern provinces of Aleppo and Idlib after residents there accused the Al Qaeda-linked group of killing a popular doctor, is the most serious since the uprising began in March 2011.
The fighting has spread to the central province of Hama as well as the northeastern province of Raqqa and has 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 the Observatory’s 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 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.