Syria rebels capture ISIS headquarters in Aleppo
Islamist rebels have captured the headquarters of the al-Qaeda-affiliated Islamic State in Iraq and the Levant (ISIS) in the Syrian city of Aleppo.
The Syrian Observatory for Human Rights said it was not clear what had happened to the jihadists who had been at the hospital in the Qadi Askar district.
Inside, the rebels found dozens of prisoners and the bodies of several men who appeared to have been executed.
Recent days have seen fierce fighting between ISIS and other rebel groups.
More than 270 people, including 46 civilians, have been killed in the worst rebel-on-rebel violence since the uprising in Syria began in March 2011, according to the Syrian Observatory, a UK-based activist group.
'Save the battlefields'
Attacks on fellow rebels and the abuse of civilian opponents of President Bashar al-Assad's government by ISIS's predominantly foreign fighters have led to increasingly frequent confrontations in recent months.
The latest clashes erupted on Friday when rebels led by the Islamic Front, a relatively new coalition of Islamist groups, launched what appeared to a series of co-ordinated strikes against ISIS in northern and eastern Syria. The offensive was backed by the National Coalition.
On Wednesday, the Syrian Observatory reported that ISIS's main base in Aleppo, a former children's hospital, had been captured by several Islamist brigades, adding that it was unclear what had happened to the "hundreds" of fighters who had been there.
However, dozens of their prisoners were reportedly found and freed.
Heavy fighting was also reported on Wednesday to the east in the city of Raqqa, the only provincial capital to be controlled by rebel forces.
Overnight, a car bomb was detonated near a rebel base in the town of Tal Abyad, next to the Turkish border to the north of Raqqa, the Syrian Observatory said.
Elsewhere, dozens of ISIS fighters were withdrawn from Deir al-Zour to reinforce the group in Raqqa, and 100 others surrendered in Aleppo's Saliheen district, it added.
On Tuesday evening, an ISIS spokesman warned its rivals that it would "crush them completely and kill the conspiracy in its cradle".
Abu Mohammed al-Adnani also said it considered members of the opposition alliance, the National Coalition, and the Supreme Military Council of the Western-backed Free Syrian Army as "legitimate targets".
"Kill them wherever you find them and without dignity," he said. "They launched this war against us and started it."
His audio statement was published hours after the head of the other al-Qaeda affiliate operating in Syria, the al-Nusra Front, called for a ceasefire, an exchange of prisoners and the establishment of an Islamic committee to mediate disputes.
Abu Mohammed al-Julani said the rebel infighting "risks costing us dearly on the ground if it continues" and urged all rebels to "give priority to the fight against the regime".
"Some groups may agree to this and others are waiting until other sides agree, and other sides are putting off a response," he said. "There is still an opportunity to save the battlefields."
Al-Nusra is believed to have been created in mid-2011 with the help of a jihadist militant umbrella group in Iraq that includes al-Qaeda, the Islamic State of Iraq (ISI).
In April 2013, ISI leader Abu Bakr al-Baghdadi announced the merger of his group and al-Nusra, creating the Islamic State in Iraq and the Levant (ISIS). But the move was rejected by Julani and al-Qaeda's overall leader, Ayman al-Zawahiri, who recognised al-Nusra as its Syrian offshoot.
Since then, ISIS and al-Nusra have operated as separate entities, with the latter focusing on toppling Mr Assad and maintaining better relations with other Islamist rebels. ISIS has seemed to be more concerned by territorial gains and implementing its extreme interpretation of Islamic law.