BEIRUT: Fighters from an Al-Qaeda-linked group expelled rival rebels from a northern Syrian town after heavy clashes Monday, then quickly moved to eliminate any pockets of resistance by setting up checkpoints on major roads and conducting house-to-house raids in search of opponents, activists said.

The Islamic State of Iraq and the Levant's seizure of the town of al-Bab is part of a brutal battle between the Al-Qaeda-affiliated group and rebels from Islamist and more moderate factions that has raged across opposition-held territory in northern Syria for the past 11 days. The rebel-on-rebel clashes are the most serious since the Syrian civil war began, and have further muddied an already complicated conflict less than two weeks ahead of a planned international peace conference for Syria in Switzerland.

The pace and scale of the rebel war-within-a-war has eclipsed even that of the opposition's fight against President Bashar Assad, with at least 700 people killed since the infighting began Jan. 3, according to the Britain-based Syrian Observatory for Human Rights. Many civilians have welcomed the fight against the "Islamic State," which alienated large chunks of the public with brutal tactics such as kidnappings, extortion and beheadings as it sought to impose its strict interpretation of Islamic law.

Fighters from the "Islamic State" captured al-Bab on Monday morning and quickly moved to solidify their grip on the town northeast of the city of Aleppo, the Observatory and the Aleppo Media Center activist group said.

"The Islamic state is going house by house to check who is a rebel fighter, who is a civilian," said Observatory director Rami Abdurrahman. "They have arrested tens of rebel fighters and civilians supporting them."

The Observatory said fighters from the extremist group used mosque loudspeakers to call on people to hand over their weapons, and told residents that they came to apply Islamic law. At the same time, the group also set up checkpoints on roads into and out of the city, and its gunmen were scrutinizing the IDs of travelers passing through.

The infighting has been ruthless, and there have been several reported mass killings of detainees, including last week at a hospital in Aleppo.

The latest reported case is in the adjacent province of Raqqa, where the "Islamic State" killed at least 46 fighters from a rival ultraconservative Islamist brigade known as Ahrar al-Sham, Abdurrahman said. He said the killings occurred near the village of Kantari about 80 kilometers (50 miles) north of the provincial capital of Raqqa city.

On Sunday, Ahrar al-Sham on its official Twitter account accused the "Islamic State" of killing some 100 Ahrar detainees in Raqqa.

The rebel infighting comes ahead of an international conference planned to convene Jan. 22 in Montreux, Switzerland. The gathering aims to broker a political solution to the Syrian civil war. But the prospects for success at the peace talks appear dim, and it remains unclear whether they will indeed take place.

Assad has flatly rejected the idea of handing over power in the talks, while the Western-backed opposition in exile, known as the Syrian National Coalition, is in disarray and not yet decided whether it will attend the gathering. Even if it does, it is in no position to wrest concessions from Assad, whose forces have seized the momentum in recent months.

At the start of two days of meetings Sunday in Paris, top envoys from 11 countries that support the Coalition were pressuring the group to attend the peace conference, saying the talks were the only way to end the carnage.

Russian Foreign Minister Sergey Lavrov and U.S. Secretary of State John Kerry said in Paris on Monday that they were pressing for a cease-fire and prisoner exchange between the warring sides.

Diplomats have struggled for months to bring Assad and his opponents to the negotiating table, while the violence inside Syria has raged on, killing more than 120,000 people, forcing over 2 million more to flee the country and destroying the economy, infrastructure and social fabric.

In the central city of Homs, mortar rounds slammed into the pro-government Ghouta and Karm al-Shami neighborhoods on Sunday, killing at least 19 people, the SANA state news agency and the Observatory said.

SANA blamed "terrorists" for the attack, the term the government uses to describe those trying to topple President Bashar Assad.

Syrian rebels often lob mortar rounds into pro-government neighborhoods of cities and towns, while President Bashar Assad's forces indiscriminately strike rebel-held areas with artillery, airstrikes and surface-to-surface missiles.