The Islamic State (Isis) have crucified 17 young men in 48 hours in retaliation for a wave of assassinations targeting their fighters.

The executions carried out by the jihadist group, which controls vast swathes of land straddling Syria and Iraq, took place in the eastern and northern Syrian cities of Deir Ezzor (or Deir al-Zor) and Raqqa "to send a message to all their opponents after recent assassinations of 12 Syrian, Iraqi and Algerian jihadists," said London-based Syrian Observatory for Human Rights.

The men, who were shot dead and then crucified for three days, were accused of fighting against Islamic State.

"Isis is sending a message to all people living under its control, to say: 'This is what will happen to any opponent'", the observatory said.

A man from the Ma'dan area in the countryside of Raqqa, the headquarters of the Sunni militant group, was executed for "taking a picture of an Isis fighter and publishing it on Facebook".

Another man, a university student in the town of al-Bolel, was accused of being an informant for the Assad regime and executed. He was initially arrested on charge of "smoking" as he returned from his university, which is located in regime-held areas.

Among those executed were five members of the Shaitat tribe, which attempted a rebellion against Islamic State rule in Deir Ezzor's Abu Hamam.

The jihadists crushed the revolt in three days, killing, beheading and crucifying members of the tribe. Around 900 people were estimated to have been killed in the bloodiest single atrocity committed by Islamic State in Syria.

Resentment among the population against Islamic State has increased in the past few months, with targeted assassinations taking out members of the group.

A senior member of Islamic State's feared al-Hesbah (or Hisbah) force in Deir Ezzor was found with his head cut off and a cigarette in its mouth. The corpse showed signs of torture and carried the message "This is evil, you Sheikh" written on it. It is unclear who carried out the decapitation but the message was obvious.

Islamic State's ban on cigarettes is one of its signature polices. It has imposed a strict set of Sharia laws barring the use of drugs, alcohol and cigarettes in the territories it has conquered.