The bodies included five middle-aged men in orange jumpsuits, the kind typically worn by Daesh hostages, as well as three women who were apparently stoned to death, the Britain-based SOHR said.
Two hundred corpses, including those of people believed to have been executed by Daesh, were found near the Syrian city of Raqqa, a local official and a war monitor said on Wednesday.
The mass grave contained the bodies of five middle-aged men in orange jumpsuits of the kind typically worn by Daesh hostages, Yasser al Khamees and the Britain-based Syrian Observatory for Human Rights said.
"They were shackled and shot in the head," said Khamees, who heads a team of first responders.
They were believed to have been killed more than two years ago, he said, adding that his team was not immediately able to identify them.
The grave also included the bodies of three women who were believed to have been stoned to death, Khamees and the Observatory said.
"Their skulls were severely fractured and displayed signs of stoning," the local official added.
The digger said his team first discovered the mass grave early last month on the southern edges of Raqqa, Daesh's former Syria stronghold.
As many as 800 people could be buried there in total, he said.
Its discovery could help identify even more of the several thousand people whose fates remain unknown, including foreigners imprisoned by Daesh.
The terrorist group took full control of the city of Raqqa in early 2014 and made it the de facto Syrian capital of its infamous so-called cross-border "caliphate."
US-backed forces ousted Daesh from the devastated city in October 2017, leaving the Raqqa Civil Council (RCC) to run it.
The RCC has been retrieving bodies from the rubble across Raqqa, left in ruins by the months-long assault to oust Daesh.
In February, an exhumation team uncovered a mass grave holding an estimated 3,500 people in Raqqa's Al Fukheikha agricultural suburb – the largest to date.
Several other mass graves have already been identified around the city, including one in the neighbourhood known as "Panorama," from which more than 900 bodies were exhumed.