The head of a US-based Syrian human rights organization reported that a mass grave outside the capital, Damascus, contains at least 100,000 bodies of people killed by the government of former President Bashar al-Assad.

Moaz Mustafa, head of the Syrian Emergency Organization, said in a telephone interview with Reuters from Damascus that the site in Qutayfah, 40 kilometers north of the Syrian capital, was one of five mass graves he had identified over the years.

He added, "One hundred thousand is the most conservative estimate" for the number of bodies buried at the site. “It is a very conservative estimate and almost unfair.”

Mustafa said that he is certain that there are more mass graves than at the five sites, and that the Syrian dead include American and British citizens and other foreigners.

Reuters was unable to confirm Mustafa's claims.

It is estimated that hundreds of thousands of Syrians have been killed since 2011, when Assad's crackdown on protests against his rule turned into an all-out civil war.

Syrians, human rights groups, and the governments of Assad and his father Hafez, who preceded him in the presidency and died in 2000, are accused of committing widespread extrajudicial killings, including mass executions, inside Syria's notorious prison system.

Assad has repeatedly denied that his government committed human rights violations and described his critics as extremists.

Syria's ambassador to the United Nations, Qusay Al-Dahhak, has not yet responded to a request for comment. He took office in January while Assad was still in power, but told reporters last week that he was awaiting instructions from the new authorities and would continue to "defend and work for the Syrian people."

Mustafa arrived in Syria after Assad fled to Russia and his government collapsed in the face of a lightning attack by the opposition, ending his family’s rule that had lasted more than 50 years.

He spoke to Reuters after an interview conducted with the British program (Channel 4 News) at the site in Qatifah to prepare a report on the alleged mass grave there.

He said that the intelligence branch of the Syrian Air Force was “responsible for transporting bodies from military hospitals, where they were collected after their owners were tortured to death, to various intelligence branches, before sending them to the mass grave site.”

He added that the bodies were also transported to the sites by the municipal funeral office in Damascus, whose employees helped transport them from refrigerated trucks.

Mustafa said, “We were able to talk to the people who worked in these mass graves who fled Syria on their own or we helped them to flee.”

He added that his group spoke to bulldozer drivers who were forced to dig graves and “often, upon orders, crush the bodies before they covered them with dirt.”

Mustafa expressed concern about the lack of security of mass grave sites and said they must be preserved to protect evidence for investigations.