Zahran Alloush, the commander of an organised, armed group in the Damascus area, has been killed in an air strike, with conflicting claims about who carried out the attack.
Alloush, head of the Jaysh al-Islam, was killed along with five other commanders in an air raid on Friday that targeted the rebel stronghold of Eastern Ghouta.
The Syrian government claimed responsibility for the operation that killed Alloush. However, some sources told Al Jazeera that Russian fighter jets were responsible.
Al Jazeera's Hashem Ahelbarra, reporting from Gaziantep in Turkey, said Alloush's death was a blow to the Syrian opposition given that he commanded a 20,000-strong "well-armed rebel force".
"He was always seen as the immediate threat to President Bashar al-Assad," he said.
"His death would be reverberating across Syria."
Al Jazeera has learned that Issam al-Buwidani has been named as the successor of Alloush.
Alloush's death could threaten the future of the peace talks as it could signal that Russia and Syria are "not genuine" in pursuing negotiations, our correspondent said.
As the leader of Jaysh al-Islam, Alloush was considered a Salafist preacher who was supported by Saudi Arabia.
He was also accused of being involved in the abduction of Razan Zaytouneh, a Syrian human-rights campaigner.
|Abu Hammam Bouwaidani has been named as Alloush's successor
Rami Abdul Rahman, director of the Syrian Observatory for Human Rights, said Eastern Ghouta was targeted by Russian air strikes on Friday.
Meanwhile, Russian jets also struck a maternity hospital in Azaz, in Aleppo province, killing at least 14 people and injuring several more, Al Jazeera's Milad Fadel reported on Friday from the city, near the border with Turkey.
"This is not the first time this hospital has been hit," he said.
He said many of those injured in the attack are being treated at the destroyed hospital. The raid also hit a nearby petrol station.
The air strikes have increased in frequency since a Russian fighter jet was shot down by Turkey near the border in November.
In Moscow, the Russian defence ministry said its continuous bombing campaign against the Islamic State of Iraq and the Levant (ISIL) group in Syria had cut down oil smuggling into Turkey.
Citing intelligence data, Russia said the number of oil tankers moving along the so-called northern route towards a refinery in the Turkish city of Batman had gone down.
Over the past week, Russian attacks have destroyed 17 truck columns as well as 37 targets involved in the extraction and refining of oil, the Russian statement said.
Overall, about 2,000 oil tank trucks have been destroyed since the bombing campaign in Syria commenced on September 30, Russia said.
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Since its start, Russian armed forces have conducted 5,240 sorties in Syria, the defence ministry said.
In other developments in Syria, a Hezbollah TV station said on Friday that in the refugee camp of Yarmouk, at least 2,000 ISIL fighters were expected to be evacuated under a deal.
The deal marks a success for the government of President Bashar al-Assad, increasing its chances of reasserting control over a strategic area adjoining the south of Damascus.
It also highlights the increasing efforts of the UN and foreign governments to bring about local ceasefires and safe-passage agreements as steps towards the wider goal of ending Syria's war, in which more than 250,000 people have been killed in nearly five years of fighting.
The besieged fighters in Yarmouk also include members of al-Nusra Front, al-Qaeda's offshoot in Syria.
Hezbollah's Al Manar TV said 18 buses had arrived to start taking them and 1,500 family members to areas under the control of ISIL and other armed opposition groups.
It was not clear whether the buses were provided by the UN or by the Syrian army.
The fighters' capitulation was forced by a government siege over several years that squeezed the flow of food and humanitarian aid, starving many people to death in what Amnesty International described as war crimes.
Source: Al Jazeera and agencies