The head and several leaders of one of Syria's most powerful rebel groups, Jaysh al-Islam, have been killed in an air strike east of Damascus.
Founder Zahroun Alloush, 44, was among those killed when rockets hit a meeting place, rebels and the Syrian army said.
The Saudi-backed Islamist group is one of the biggest factions and is dominant in the Eastern Ghouta countryside.
It recently joined an opposition summit in Riyadh which produced a framework for peace talks with the government.
Ten rockets struck as Jaysh al-Islam commanders met, Saudi-funded al-Arabiya TV reported. The group's deputy leader was also killed, al-Arabiya said.
The killing of Zahran Alloush comes days after the UN passed a resolution endorsing a path to peace in Syria.
It sends a strong message as to whom the Russians and the Syrian government are willing to sit at the negotiating table with, when and if peace talks take place.
Russia, President Assad's strong ally, has presented a list of rebel groups it identifies as terrorists. Jaysh al-Islam was one of them.
Jaysh al-Islam controls most of the Eastern Ghouta agricultural belt on the edge of Damascus.
It succeeded in forcing so-called Islamic State out of the area, which has been under heavy bombardment from the regime since early on in the war.
In a statement carried by state television, Syria's army command said it had conducted the "special operation" that killed Alloush, reported AFP news agency.
However, some activists said it was a Russian air strike, while other sources suggested the Syrian air force had conducted the strike using Russian missiles.
Analysts called it a severe blow for rebel forces and a threat to the nascent efforts to find a political resolution.
Russia, a key ally of Syrian President Bashar al-Assad, has been waging an air campaign in Syria since the end of September.
Moscow insists it has been targeting so-called Islamic State, but rebels and Western officials say the Russian strikes have mainly been hitting other groups.
Jaysh al-Islam, whose fighters number tens of thousands, took part in the conference in the Saudi capital which agreed a common approach among disparate rebel groups for UN-backed peace talks planned for January.
Syria and Russia dismissed the meeting, saying the groups did not properly represent the opposition and that those that did attend were unacceptable.
