The Syrian Observatory for Human Rights, an NGO that has become a key source of information on the war in Syria, said Monday it had received death threats from Islamist extremists.
"Recently, we have received a large number of death threats... on the SOHR (Facebook) page and the (Skype) and email accounts of numerous members and activists," the group said in an email.
Observatory director Rami Abdel Rahman told AFP that the threats originated from Islamist extremists, declining to specify which ones.
"These groups are trying to locate our sources in the regions under their control," he said, particularly in the north and east of the country, as well as in parts of Damascus province.
"These sources are being threatened," he said.
Since the beginning of the Syrian conflict in March 2011, the Britain-based Observatory has relied on a network of hundreds of sources, including activists, doctors and lawyers, but also members of the security forces, to give a daily toll in the bloody war.
Abdel Rahman said the group has been threatened before, but the latest threats appeared to be the most serious, prompting him to go public with them.
He said they began after his group published information last week about the deaths of dozens of Islamist rebels, killed by regime fire in an area near Damascus.
"To all those parties who accuse us of falsifying the facts, saying that those we said were Islamist rebels were civilian martyrs... we urge you to publish their names or photos of these civilians," the Observatory said in an email.
"Is it not astonishing that several days after the incident, there have been no photos or names of a single child or a single woman among the dozens supposed to have died in the shelling?"
The Observatory, set up in 2006, has become a key source in the death toll in Syria's conflict.
Its reporting is followed closely by the international media and diplomatic corps, particularly as journalists have increasingly avoided rebel-held areas because of the risk of kidnapping by Islamist extremists.
In an interview with AFP earlier this year, Abdel Rahman warned that while the Syrian regime "commits dozens of atrocities every day," Islamist extremists were also committing abuses and were trying to set up an "alternative dictatorship."