Jabhat Fateh al-Sham has taken over areas held by a Free Syrian Army faction in north-western Syria, FSA officials said on Wednesday.
Jabhat Fateh al-Sham or JFS was known as the Nusra Front before it claimed to have severed ties with al Qaeda in 2016.
It was accused of launching a surprise attack on the FSA in areas of Aleppo and Idlib on Tuesday as the Astana talks were wrapping up in Kazakhstan. The Syrian Observatory for Human Rights, a Britain-based monitoring group, said five civilians died in the clashes.
Reacting to what it called "humiliating" decisions taken by Turkey, Iran and Russia and the Syrian opposition leaders in Astana, a JFS statement on Tuesday said the negotiations were trying to divert the revolution "towards reconciliation with the criminal regime" of Bashar al Assad.
Russia also gave rebels a draft version of a new constitution for Syria, Russia's envoy for Syria Alexander Lavrentiev said on Tuesday. "We did this exclusively to accelerate the process" to end the war.
The rebels said they refused to discuss the draft with Moscow.
"We didn't even pick it up," a source from the Astana delegation representing the rebel forces said.
Strikes on Daesh
Six Russian Tupolev strategic long-range bombers struck Daesh targets in Syria's Deir al-Zor province on Wednesday, Russia's Defence Ministry said.
The long-range supersonic bombers delivered the strike after taking off from aerodromes in Russia and flying over Iraq and Iran, the ministry's press office said.
"Two militants' command posts, ammunition, arms and military hardware depots were the air strike's targets," the ministry said in a statement obtained by Russian news agency TASS.
The Syrian regime also took on Daesh, gaining more territory east of Aleppo, according to the regime and a statement on the SOHR website on Wednesday.
The regime took over at least four more villages along the way to al-Bab as part of an offensive which started mid-January.
Syrian rebels, backed by Turkish special forces, tanks and warplanes, have been besieging al-Bab since December to rid the town of Daesh.
It is not certain what will happen when regime troops reach al-Bab, where the rebels and Turkish forces have a presence.