BEIRUT (AP) — A wave of tit-for-tat kidnappings between rival Islamic militant groups in the northern Syrian city of Aleppo risks sparking large-scale internal fighting between rebels after clashes killed at least four militants earlier this week, activists said Saturday.

The director of the Britain-based Syrian Observatory for Human Rights, Rami Abdul-Rahman, said a coalition of rebel groups known as the Judicial Council had accused another armed opposition faction, the Ghurabaa al-Sham, of plundering factories in Aleppo's industrial neighborhood. Aleppo, Syria's largest city and a former commercial center, is split between rebel and government control.

Any internal fighting between rebels in the city would play into the hands of the regime, which is trying to tarnish the image of the opposition by saying it is dominated by extremists linked to al-Qaida network.

Aleppo, a city of 3 million that was once a bastion of support for President Bashar Assad, has been engulfed in heavy fighting since rebels launched an assault there in July and captured several neighborhoods. Over the past few weeks, regime forces have been pursuing an offensive in the city, mainly focused on pushing the rebels from around the international airport and a nearby military air base.

Abdul-Rahman said tensions among rebel factions have been rising in opposition-held areas, mostly on the eastern side of the city.

The two groups, the Judicial Council and the Ghurabaa al-Sham, clashed on Tuesday near Aleppo in fighting that left four members of the Judicial Council dead, Abldul-Rahman said. He added that the Judicial Council is now holding dozens of members of Ghurabaa al-Sham captive.

Aleppo-based activist Mohammed Saeed said Ghurabaa al-Sham withdrew its fighters from several neighborhoods, including the industrial area, and that it had released all of the Judicial Council members it had been holding captive.

"The situation is very tense in Aleppo," said Abdul-Rahman, who relies on a network of activists around the country. He said that Ghurabaa al-Sham has warned it will bring some of its members from outside the city to fight against the Judicial Council if its members are not freed.

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Saeed said Ghurabaa al-Sham released all Judicial Council members it was holding while the other group refused to set free Ghuarbaa al-Sham members and is still holding them.

He added that the Judicial Council is an umbrella organization that includes the Tawheed Brigade, al-Sham Liberals and the al-Qaida-affiliated Jabhat al-Nusra — one of the most effective forces among the mosaic of rebel brigades fighting to topple Assad in Syria's civil war.

"There are fears that fighting (between rebels) might erupt in Aleppo," Saeed said by telephone.

In other parts of Syria, the Observatory reported that rebels captured several villages late Friday in the central province of Hama after weeks of fighting with government troops. It said the villages were inhabited by members of Assad's minority Alawite sect, an offshoot of Shiite Islam.

It said the Alawite villages — Tleisiyeh, Zaghba, Shaata and Balil — are all on the eastern side of the central province. The Observatory said residents fled the area captured by rebels.

The uprising against Assad's rule that began in March 2011 quickly became an outlet for long-suppressed grievances, mostly by poor Sunnis from marginalized areas. It has since escalated into an outright civil war that killed more than 70,000 people according to the United Nations.

The conflict has grown increasingly sectarian, both in action and rhetoric.

Earlier this month, activists reported that troops and pro-government Alawite gunmen killed more than 100 people in Sunnis areas in the coastal city of Banias and the nearby town of Bayda. The violence in Banias and Bayda bears a close resemblance to two reported mass killings last year in Houla and Qubeir, Sunni villages surrounded by Alawite towns.

Many of the rebels trying to overthrow Assad today say they want to replace his government with an Islamic state.

The Syrian National Coalition, the main umbrella opposition group warned in a statement that government forces are currently imposing a siege and communications blackout on the towns of Halfaya and Aqrab in Hama.

"Civilians in those areas are now cut off from contact with the outside world, and lives are in extreme danger," the coalition said in a statement.

The Observatory and the Local Coordination Committees, another activist group, reported intense clashes around the town of Qusair near the Lebanon border. Syrian opposition groups say members of Lebanon's militant Hezbollah group are taking part in the fighting along with Assad's forces.