"The Democratic Union Party (PYD) is fighting in the Sinjar area and Rabia," a senior army official told AFP, stressing that "it was not coordinated with the Iraqi government."

The PYD, whose armed wing has been fighting Islamic State jihadists in Syria and become the dominant force in Syria's Kurdish regions, posted pictures of its forces in the Rabia area.

When IS fighters conquered Iraq's second city of Mosul nearly two months ago and swept through much the country's Sunni heartland, Kurdish peshmerga forces moved into several towns and key sites abandoned by government forces.

On Sunday, the jihadists attacked peshmerga positions, forcing them to withdraw and sparking a mass exodus among civilians, many of whom belong to Iraq's most vulnerable minorities.

The move allowed IS secure much of Mosul's hinterland, take control of two small oil fields, and further abolish the border between the two halves of the "caliphate" straddling Iraq and Syria which their leader Abu Bakr al-Baghdadi proclaimed in late June.

Iraq's peshmerga and the PYD have had tense relations in the past but the Syria group posted pictures on the Internet of what it said were its forces operating in Rabia, inside Iraq.

"The PYD has gone to Sinjar based on a request from the people. Our units reached Sinjar and on their arrival fierce fighting erupted with Daash," the group said on its Twitter account, using IS' former Arabic acronym.

The peshmerga announced Monday that it has already launched a counter-offensive to recapture the ground it lost at the weekend but did not mention the PYD's presence.