London, May 28 (IANS/AKI): Islamic State terrorists shot dead 20 people in the Roman Theatre of the ancient Syrian city of Palmyra, forcing local people to watch the executions, a monitor said.

IS claimed those executed were Shia and Alawite Muslims from the Syrian armed forces and allied militiamen, said the UK-based Syrian Human Rights Observatory.

Local activists in Palmyra said on Facebook the people executed inside the Roman Theatre were "Asians" and "Iranians" and that local people were allowed to watch.

Neither the Syrian government or IS have confirmed the executions.

Since it overran the ancient city 10 days ago, IS has executed at least 237 people in Palmyra, including dozens of children and male and female civilians, as well as pro-government forces, the Observatory said.

IS had also taken around 600 people prisoner since it seized control of Palmyra, including civilians and pro-government forces accused of being "agents of the regime," the watchdog said.

It relies for its information on a network of activists and doctors inside Syria.