The man was an Egyptian national and was known as the deputy "emir" of the al-Hesbah force in a Syrian province, the British-based Syrian Observatory for Human Rights said today.
His body, which showed signs of torture, was found near a power plant in al-Mayadeen city in the Deir-al-Zor province, it said, citing contacts on the ground. "We do not know whether Islamic State killed him or whether it was local people or other fighters," said Rami Abdulrahman, who runs the Observatory, which reports on events in Syria through a network of contacts on the ground.
"Either way it is important, because he was a very important man," he said, adding that he believed it was the first time that an al-Hesbah member had been killed in this way.
Islamic State, an offshoot of al-Qaeda which has drawn in foreign fighters and is also known as ISIL or ISIS, has seized land in Syria and neighbouring Iraq. It has been the target of US-led air strikes in both countries since September.
The Observatory said the message "This is evil, you Sheikh" was found written on the corpse, which had a cigarette in its mouth. Residents in areas controlled by Islamic State have said the group has banned smoking in public.
Islamic State has fought with other insurgents and cracked down on local populations. It has also killed its own members for what it describes as violations.
Residents and activists say it has beheaded and stoned to death many people in areas it controls for being enemy fighters or for actions they see as violating their reading of Islamic law, such as adultery and blasphemy.
In December, a similar self-declared police force in western Syria decapitated four men after accusing them of blasphemy, according to the Observatory. It reported a similar killing days earlier in the north of the country.
The Observatory also reported today that unknown assailants also tried to kill two Islamic State militants in al-Mayadeen city. The first attempt was when a car tried to run over a fighter near a roundabout. Another was hit by an attacker carrying a metal weapon and travelling by motorcycle, and was seriously wounded