In last months there has been a spike in guerrilla-style attacks by groups of Syrians against ISIS fighters inside their strongholds in Raqqa and Deir al-Zor provinces, Reuters reports.

The Syrian Observatory said the man was beheaded and his body crucified in a public park in the town with his head left displayed for three days.

He was accused of "setting up a cell to fight ISIS" by staging ambushes and detonating vehicles of fighters of the militant group, it said.

Moreover four other men, one a student, were executed in Deir al-Zor for alleged ties with the Syrian authorities.

"One was arrested for smoking a cigarette. Only later did ISIS accuse him of being an informer for the Syrian government," Observatory head Rami Abdulrahman said. Islamic State bans smoking in areas under its control in line with its strict interpretation of law.

Syria's state news agency SANA said ISIS executed three civilians and displayed their bodies in al-Mayadin. It did not say what their crimes were.

Small groups of Syrians who are hunting down ISIS fighters say it is part of a guerrilla campaign that has emerged as a response to the group's growing brutality.

ISIS controls nearly all of Deir al-Zor province, which stretches from Raqqa to the border with Iraq and links together its self-declared caliphate in the two countries.

The leader of one such guerrilla group, "White Shroud", says his group has killed more than 100 ISIS fighters in attacks in Deir al-Zor province in recent months.

ISIS crushes dissent in Deir al-Zor, where its control expanded rapidly after it seized the Iraqi city of Mosul in June. ISIS executed 700 members of one rebellious tribe, the Sheitaat, the Observatory reported in August.