Syria's leading opposition body blamed the government for the mysterious death of one of its members after a car accident in the capital Damascus.
Munir Darwish, 80, was a writer and founder of a Cairo-based opposition group seen as "tolerated" by president Bashar Al Assad's government.
He joined the mainstream Syrian Negotiations Committee (SNC) in November but continued to live in Damascus.
The SNC said on Saturday that Darwish was the victim of "a hit-and-run outside his home in Damascus, followed by a premeditated elimination [killing]" on Friday night.
"We hold the tyrannical Assad regime responsible for Munir Darwish's death," the SNC said in a statement, referring to the incident as an "assassination".
Firas Al Khalidi, who heads the Cairo platform of the opposition, told AFP that the exact cause of death remained unclear.
After the car accident, Darwish underwent ankle surgery at the capital's Mawasat Hospital and was in "excellent health", awaiting release on Saturday, he said.
"At around midnight on Friday, they called to say he's dead. I found out from his son," he said.
Mr Khalidi said Darwish had not left Syria since late November out of fear for his safety, even though both his sons and wife lived abroad.
"He clearly hinted that there were threats," Mr Khalidi added, without directly accusing the government.
"Who has an interest in his assassination, except those who hate Syria -- with the regime at the top of the list?" Mr Khalidi said.
A statement from the office of Syria's UN envoy Staffan de Mistura said he was "shocked and saddened" to hear of Darwish's death.
"The Special Envoy calls for those involved to be identified and brought to justice," it said.
Syria's conflict erupted in March 2011 with widespread protests against the Assad regime but it has since morphed into a brutal and complex war.
Numerous rounds of UN-brokered peace negotiations held in Geneva have failed to end the war, which has left more than 340,000 people dead.