CAIRO (AFP) - The Egyptian branch of the Islamic State group has posted pictures online of what it says was the execution of two men caught spying for the army in the Sinai.

One of those decapitated was presented by the ISIS offshoot Sinai Province as a "spy for the military intelligence" services, while it said the other was a "spy for the army".

The images which were distributed on Twitter on Thursday night were deemed authentic by US monitor SITE Intelligence Group.

The ISIS affiliate is waging an insurgency in the restive peninsula that has killed hundreds of soldiers and policemen since the military overthrew Islamist president Mohamed Morsi in 2013.

The militants say they caused the crash of a Russian passenger plane carrying holidaymakers over Sinai last October, killing all 224 people on board.

ISIS said it had smuggled a bomb onto the plane at an airport in the south of the peninsula.

The Sinai militants pledged allegiance in November 2014 to ISIS, which controls parts of Iraq and Syria.