Beirut (AFP) - Islamic State fighters executed a youth in a Syrian town Friday, after hundreds of residents demanded they leave following regime air strikes that targeted the jihadists but killed eight civilians, a monitor said.

Residents of Ashara, in the mostly IS-controlled eastern province of Deir Ezzor, protested in front of an IS headquarters Thursday evening, hours after air strikes killed two children, five women and a man, Syrian Observatory for Human Rights director Rami Abdel Rahman said.

In response, IS gunmen opened fire and seized several young men, the Observatory and an activist said.

The air strikes were part of a stepped-up campaign launched in recent weeks by President Bashar al-Assad's regime against IS-held towns and villages in eastern and northern Syria.

They resumed Friday, with four raids targeting jihadist positions in Deir Ezzor city, the Observatory said, adding that there was no confirmation of any casualties.

Activists have frequently condemned the attacks for killing not only jihadists, but also civilians.

"Thursday evening's protest (in Ashara) began as a funeral for the victims, attended by nearly 300 people," said Rayan al-Furati, an activist from Deir Ezzor.

"Then the mourners began to protest, demanding that the IS leave the town of Ashara," Furati told AFP via the Internet.

The Observatory said that, on Friday, the IS publicly executed and crucified a young man identified as Ali Khalaf, accusing him of "heresy and apostasy."

"Though he had nothing to do with yesterday's protests, they executed him publicly in Ashara in order to terrorise people into not taking any kind of action against the IS," Abdel Rahman said.

Furati said the jihadists, whose abuses and radical interpretation of Sunni Islam have been widely documented, are vastly unpopular. But, he added, people living under their control are deeply fearful of taking action against them.

"It's like living in a prison," Furati said, using a pseudonym to protect his family from retaliation.

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"Not a single day passes without an execution somewhere in Deir Ezzor province."

Elsewhere, the Observatory said loud explosions were heard near an IS base in Raqa city, which the jihadists refer to as their "capital," following air strikes in the area.

The Syrian conflict began as a peaceful movement demanding Assad's ouster, but then morphed into a brutal civil war after a massive regime crackdown against dissent.

In June, IS declared an Islamic "caliphate" straddling the territory it controls in Syria and Iraq.