A mass grave has been found outside the capital Damascus containing at least 100,000 bodies of people killed by the regime of ousted President Bashar al-Assad, the head of a U.S.-based Syrian rights group said.

The site in Qutayfah, 40 km (25 miles) north of the Syrian capital, was one of five mass graves he had identified over the years, Moaz Mustafa, head of the Syrian Emergency Organisation, told Reuters in a telephone interview from Damascus.

Mustafa considered that "100,000 is the most conservative estimate" of the number of bodies buried at the site. "It is a very conservative estimate and almost unfair," he said, adding that he was certain that there were more mass graves than the five sites, and that in addition to the Syrian dead there were American, British and other foreign victims.

Mustafa returned to Syria after Assad fled to Russia and his government collapsed in the face of a lightning opposition offensive that ended his family's more than 50-year rule.

He spoke to Reuters after being interviewed by Britain's Channel 4 News at the site in Qatif, to report on the alleged mass grave there.

He said the Syrian Air Force Intelligence Branch was "responsible for transporting the bodies from military hospitals, where they were collected after being 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 in refrigerated trucks.

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

He added that his group spoke to bulldozer drivers who were forced to dig graves and "often, on orders, crush the bodies before covering them with dirt."

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

Hundreds of thousands of Syrians are estimated to have been killed since 2011, and Syrians and human rights groups accuse the regime of ousted President Bashar al-Assad and his father, Hafez, who preceded him in the presidency and died in 2000, of carrying out widespread extrajudicial killings, including mass executions inside Syria's notorious prison system.

Syria’s UN ambassador, Qusay Dahhak, did not immediately respond to a request for comment. He took up the post 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.”