BEIRUT (AP) - An al-Qaida group fighting in Syria against rebels trying to dislodge it from opposition-held parts of the country has killed at least four activists it held captive in the area, their colleagues said Tuesday. The killings came against the backdrop of clashes between rival rebel factions in Syria's northern provinces. The fighting, which has pitted an array of rebel groups against the al-Qaida-linked Islamic State in Iraq and the Levant, also known as ISIL, is now in its fifth day. The clashes are part of a widening war within a war in Syria, this one against radical extremists, adding 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. Syrian activists said the al-Qaida group killed at least four of their colleagues during fighting Monday around an eye hospital that has served as a stronghold for the extremists in the Qadi Askar quarter of the northern city of Aleppo. Three of the killed worked for an opposition outlet called "Shada al-Hurriya" and were seized in December, said the Syrian Journalists Association. An activist, who only identified himself by his nickname Abu Saleh al-Halaby because of fears for his own safety, said the fourth slain colleague was the media man for another hardline Islamic rebel brigade, the Nusra Front. Rami Abdurrahman of the Britain-based Syrian Observatory for Human Rights also reported that four activists were killed but couldn't confirm their identities. The Observatory and other activist groups rely on a network of people on the ground in Syria. The killings underscore concerns for the fate of unknown numbers - at least dozens - of Syrians and foreigners believed held by the al-Qaida-linked group. Since muscling in into rebel-held areas of the country in March, the al-Qaida fighters are believed to have detained foreign reporters, Syrian activists, rival rebel fighters and others critical of their rule. They have also detained citizens perceived to have disobeyed their extreme interpretation of Islamic law. Simmering anger against the group exploded after residents reported that the extremist fighters had tortured and killed a popular doctor in Aleppo in December. Rebel clashes escalated since, becoming the most serious rebel infighting since the uprising against President Bashar Assad began in March 2011. The Observatory on Tuesday reported that three Syrians captured months ago by the al-Qaida-linked group were found dead in the eastern Syrian town of Kafar Zeita after the radical fighters fled from there. The Observatory also said 10 Syrians, who had escaped from al-Qaida detention in the village of Deir Hassan in the northern province of Idlib, said their captors decapitated two Syrians detained there in the past week. In rebel-on-rebel fighting Monday, Syrian rebels surrounded a prison in the city of Raqqa in the east that was held by the al-Qaida fighters and freed at least 50 captives. Abdurrahman of the Observatory said most of the infighting used tit-for-tat attacks on compounds, villages and areas dominated by one group or the other. After al-Qaida fighters used suicide car bombs to strike a rebel checkpoint in the town of Derkosh in the northern province of Idlib, and another in Aleppo, the rival rebels reiterated with an attack on the al-Qaida group in Idlib, killing 30 of their fighters, Abdurrahman said Monday. The fighting adds another hardship to an already-suffering population and more uncertainty to the so-called Geneva peace conference, which is to open Jan. 22 in Switzerland to try and find a resolution to the Syrian conflict. In the city of Aleppo, water had been cut off for the past five days while municipal workers pleaded with rebels to allow safe passage to deliver flour to bakeries, reported the activist Aleppo Media Center. The Center and the Observatory said Tuesday that a government airstrike killed 10 civilians, including children, in the rebel-held northern town of Bzaa on Monday.