The Islamic State jihadist group on Thursday released pictures of six men being executed on charges of "spying" for the Iraqi government in the city of Fallujah.

In a statement posted on social media, the IS jihadists use three different methods to execute their victims, all wearing orange jumpsuits.

"The judicial police carried out death sentences issued by an Islamic court in Fallujah against several apostates after they were convicted of spying," the statement said.

The statement was written like an official government release and described the circumstances that led to the capture of each of the six men.

The pictures showed masked IS fighters in full tactical gear, against the backdrop of a heavily damaged city.

Four of the victims were shot in the head, one was decapitated with a knife and another with an explosive rope.

Fallujah lies in Anbar province only 50 kilometres (30 miles) west of Baghdad and is the jihadist group's largest remaining stronghold in Iraq after the city of Mosul.

IS controlled most of Anbar a few months ago but sweeping military operations by the Iraqi security forces backed by air strikes from the US-led coalition are turning the tide.

Fallujah is almost completely isolated from other IS-controlled territory and IS is believed to be increasingly struggling to get supplies into the city.

The situation also appears to be causing increased internal tension in Fallujah, with IS paranoid that residents are assisting with an impending government offensive.

Jihadists detained dozens of residents of the city last month after clashes between IS and Iraqi tribesmen.