The mass grave contained the bodies of five middle-aged men in orange jumpsuits of the kind typically worn by 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 de-facto capital.
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.
Daesh took full control of the city of Raqqa in early 2014. U.S.-backed Syrian Democratic Forces (SDF), dominated by the PKK terrorist group's Syrian offshoot People's Protection Units (YPG), captured the devastated city in October 2017 after a four-month campaign, leaving the YPG-linked 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 Raqa's Al-Fukheikha agricultural suburb -- the largest to date.
Several other mass graves have already been identified around the city, including one in the neighborhood known as "Panorama," from which more than 900 bodies were exhumed.
The U.N estimates that more than 10,000 buildings were destroyed or 80 percent of the city in the clashes. Amnesty International and London-based watchdog group Airwars announced in April that t he U.S.-led coalition killed more than 1,600 civilians in Raqqa during months of bombardment that liberated it from the Daesh terror group, far more than the number announced earlier by the U.S.-led coalition.
