Syrian rebels overran the Aleppo headquarters of the Islamic State of Iraq and the Levant (ISIL), as claims emerged that the Al-Qaeda linked group had massacred prisoners there in cold blood.
The rebels were on Wednesday reportedly also pressing ISIL in Raqa, the only provincial capital lost by the regime of President Bashar al-Assad and a stronghold of the group.
In Damascus, a security source told AFP the infighting benefited Syria's regime, calling it a settling of scores by nations backing different rebel groups.
The operation in Aleppo came a day after ISIL's spokesman threatened to "crush" opposition fighters who have attacked the group in several provinces.
"There are hardly any ISIL members left in the city of Aleppo," the Syrian Observatory for Human Rights said.
Elsewhere, a diplomat announced two Swedish reporters missing in Syria since November had been freed.
However, a Czech humanitarian aid group said Wednesday that three of its local staff had been killed in Syria during a mortar attack.
And the fractious Syrian opposition National Coalition said it was postponing a final decision on whether to attend peace talks in Switzerland.
On the ground, ISIL battled moderate and Islamist rebels in clashes that first erupted on Friday and have killed at least 385 people, including 56 civilians.
ISIL's headquarters in a hospital in Aleppo's Qadi Askar neighbourhood was overrun by opposition fighters, who reportedly freed dozens of prisoners.
A video posted online Wednesday claimed ISIL had previously executed at least nine prisoners after handcuffing, blindfolding and shooting them in the head.
The Coalition denounced "these acts, which perpetuate the regime's methods to kill free voice, suppress liberty and violate... fundamental human rights".
Late Tuesday, ISIL spokesman Abu Mohammed al-Adnani issued a defiant message, urging fighters to "crush them (the rebels) totally and kill the conspiracy at birth".
His message came hours after the head of Al-Qaeda's Syrian arm, Al-Nusra Front, urged an end to the fighting.
Abu Mohamed al-Jolani warned the fighting "risks costing us dearly on the ground if it continues" and urged all fighters "to give priority to the fight against the regime".
The Nusra Front was established in mid-2011 with help from ISIL's Iraqi precursor.
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The Iraqi group's chief later sought to merge with Al-Nusra, but they spurned an alliance and pledged allegiance to Al-Qaeda chief Ayman al-Zawahiri.
Since then, they have functioned separately, with Al-Nusra largely neutral in the latest fighting.
But the Observatory said Nusra fighters were pressing ISIL in Raqa, to the east of Aleppo.
Regime operations continued Wednesday, with the Observatory reporting at least eight people killed in air raids on Tal Rifaat in Aleppo province.
Russia blocked a British-drafted UN Security Council statement condemning the Syrian government attacks on the city of Aleppo, diplomats said.
It was the second time in a month that Russia objected to a western bid to slam Assad's air assault against Syria's biggest city, which has killed hundreds since December 15.
Sweden's ambassador confirmed that two journalists missing in Syria since November had been freed.
Swedish media named them as Niclas Hammarstroem and Magnus Falkehed.
At least 25 journalists have been killed in Syria since the conflict began in March 2011, and more than 30 are thought to be missing.
Both the regime and its opponents have been accused of abducting and killing journalists.
Meanwhile the Prague-based "People in Need" organisation said a group of their staff came under fire in the northern city of Aleppo on Tuesday and three were killed.
In Istanbul, a fiercely debated National Coalition meeting ended without a decision on attending the January 22 peace talks in Switzerland. The general assembly said it will make a decision next Friday.
US Secretary of State John Kerry is hitting the road again this weekend, visiting France and Kuwait for discussions about the war in Syria before the peace get underway.
And in The Hague, the world's chemical watchdog called for Syria to speed up handing over its arsenal for destruction, after missing a key deadline.
The joint OPCW-UN mission said Tuesday that a first cargo of chemicals had been brought to Latakia and transferred to a Danish vessel, but all Syria's most dangerous chemicals were supposed to have left the country by December 31.
Even so, mission head Sigrid Kaag expressed guarded optimism that the mid-year target for destroying the entire arsenal can still be met.