Islamic State (Isis) jihadists renaged on a pledge to spare the lives of rival rebel fighters who surrendered arms in an eastern Syrian city, crucifying eight of them, activists said.

The Syrian Observatory for Human Rights (SOHR) reported that the eight men were accused of "fighting IS" and executed, days after giving up fighting in the city of al-Bukamal, off the Iraqi border in the eastern Deir Ezzor province.

"The men surrendered in al-Bukamal because the Islamic State had offered amnesty to people who fought them if they turned themselves in," SOHR director Rami Abdel Rahman told AFP.

The fighters were crucified; their bodies left on public display on a roundabout, the group said.

In another similar public execution carried out by the Sunni extremists in the same area, three men were beheaded in the provincial capital - Deir Ezzor.

Two of the victims were accused of dealing with government forces.

The third was said to be a member of the so-called Awakening Movements, Sunni Muslim militias that supported US troops against Isis Iraqi-based predecessor - al-Qaeda in Iraq - in the country in 2006.

SOHR said the three were beheaded and their bodies hung on a public park's fence.

The monitor relies on a large number of informers in Syria and has been consistently reporting on the civil war since the revolution started in 2011.

IS is known for its brutality and has held public and mass executions in the recent past both in Iraq and Syria.