An Iraqi journalist has been kidnapped by Islamic State militants who are threatening to behead him if he refuses to work for them, media watchdog Reporters Without Borders (RSF) warned Thursday.

Raad Mohammed al-Azzawi, a cameraman, was taken prisoner on September 7 in Samarra, a city around 120 kilometres (75 miles) north of Baghdad, RSF said in a statement.

"The jihadist organisation has announced that it plans to carry out its decapitation threat because the journalist refused to work for the Islamic State (IS)," it added.

He joins a growing list of Iraqi journalists who have been captured or threatened by militants from IS, a radical Islamist group that has seized a swathe of Iraq and Syria and has committed widespread atrocities.

Two captive US journalists, James Foley and Steven Sotloff, were beheaded in recent weeks according to videos released by the jihadists.

"The Islamic State since its emergence has made journalists a terror target," said Virginie Dangles, assistant research director of Reporters Without Borders.

"The terrorist organisation, in setting up an apparatus for kidnapping and executing news professionals, is attempting to eliminate all those who refuse to swear allegiance to ISIS," as IS was previously known.

US President Barack Obama has ordered a "relentless" war against IS, including air strikes in Syria and expanded operations in Iraq to "destroy" the marauding jihadist army. (***)