A mass grave outside Damascus contained the bodies of at least 100,000 people killed by the former government of ousted President Bashar al-Assad, according to the head of a US-based Syrian advocacy organisation.

Reuters news agency quoted Mouaz Moustafa as saying that the site at al-Qutayfah 40km (25 miles) north of the Syrian capital, was one of five mass graves that he had identified over the years.

“One hundred thousand is the most conservative estimate” of the number of bodies buried at the site, said Moustafa, head of the Syrian Emergency Task Force. “It’s a very, very extremely almost unfairly conservative estimate.”

Moustafa claimed there are more mass graves, and that along with Syrian victims, they included foreigners, including United States and British citizens.

Hundreds of thousands of Syrians are estimated to have been killed since 2011 when al-Assad’s crackdown on protests against his rule grew into a full-scale civil war.

Al-Assad and his father Hafez, who preceded him as president and died in 2000, are accused by Syrians, rights groups and other governments of widespread extrajudicial killings, including mass executions within the country’s notorious prison system.

Al-Assad repeatedly denied that his government committed human rights violations and painted his detractors as extremists.