Beirut, Asharq Al-Awsat—Activists from the Syrian Observatory for Human Rights said on Monday that fighters from the Free Syrian Army (FSA) and Islamist factions have besieged the headquarters of the Islamic State of Iraq and Syria in Raqqa, northeastern Syria.

The Syrian Observatory for Human Rights (SOHR) also said 50 detainees were freed from an Islamic State of Iraq and Syria (ISIS) prison in the town.

French news agency Agence France-Press quoted SOHR chief Rami Abdelrahman as saying: “Prisoners include opposition fighters and activists held by ISIS, and those freed did not include the kidnapped Christian bishop or foreign journalists.”

Abdelrahman was referring to Greek Orthodox Bishop Boulos Yazigi and Syriac Orthodox Bishop Yohanna Ibrahim, who were reportedly kidnapped by foreign militants in April 2013. Their fate remains unknown, despite rumors that they were killed by their kidnappers shortly after being seized.

A military source on the FSA’s eastern front, speaking on the condition of anonymity, told Asharq Al-Awsat : “ISIS may lose control of Raqqa at any moment, exactly like the regime suddenly lost control of it.”

The source added that fighters from Ahrar Al-Sham and the Al-Nusra Front, as well as FSA brigades in Raqqa, had succeeded in taking control of several buildings previously held by ISIS, adding that “ISIS fighters retreated to the governor’s headquarters, [which were] surrounded by members of the opposition.”

Meanwhile, a leading member of ISIS who is a Belgian national of Algerian descent threatened retaliation for the attacks on ISIS in Raqqa. He was quoted by SOHR as saying: “Hundreds of suicide bombers from ISIS were prepared to blow themselves up in attacks on FSA in Idlib province.”

A number of opposition groups have recently declared war on ISIS, which the opposition has accused of having links to the Assad regime, of imposing its extremist ideology on territory seized by the opposition, and carrying out kidnappings and executions, as well as attacking members of other opposition groups and refusing to coordinate its activities with them.

Among the most prominent of these groups are the recently formed Jaysh Al-Mujahideen and the Syria Revolutionaries Front–which does not have an Islamist leaning–in addition to the Ahrar Al-Sham movement.

A military spokesman for the Islamic Front, Captain Islam Alloush, told Asharq Al-Awsat: “The Islamic Front did not take part in the FSA attacks on ISIS, but was only defending itself.”

Alloush refused to comment on the participation of Ahrar Al-Sham, one of the Islamic Front’s factions, in the battles in Raqqa.

The FSA has continued to attack ISIS in other areas in northern Syria. A senior member of the FSA’s local operations room, speaking on the condition of anonymity, told Asharq Al-Awsat: “We are 72 hours away from liberating Idlib and Aleppo, after which we will aim to liberate the border crossings with Turkey if ISIS members have not already vacated them.”

He added: “Saraqib in Rif Idlib is on the verge of being completely liberated and, therefore, ISIS will not have any major positions except in the villages of Azmarin and Dana.” He said that in Aleppo, ISIS only had control of the eye hospital and the two strongholds of Azaz and Jarablos, adding that the battle there will be hard, but will be “in our favor.”

He said: “Opposition fighters were laying siege to ISIS in the Sheikh Suleiman Barracks in Deir Ezzor, where we think a number of hostages are held, including the two bishops.”

Another FSA officer warned: “ISIS has reached a desperate point which may lead to the execution of prisoners.”

He claimed that ISIS “executed in cold blood 31 of Syria’s revolutionaries at their base at the eye hospital in Aleppo, in addition to 20 rebels in the western suburbs near Al-Dana.”

He added: “The opposition attacks on ISIS in the east, especially in Raqqa, have destroyed its morale, and hopes of fortifying its positions in Raqqa.”