"The commission finds it impossible to organise the referendum at the same time as the presidential and legislative polls because this would affect the credibility and integrity of the (electoral) process," it said in a statement.

The panel said it had proposed delaying the polls in northern Iraq until mid-August to allow the referendum to go ahead as planned but that the Kurdish parliament had rejected the idea.

It was unclear how the dispute would be resolved.

On June 24, Iraq's autonomous region passed the constitution in which it laid claim to the disputed oil-rich province of Kirkuk, a move likely to increase ethnic tension with the country's majority Arabs.

The text, which also said that areas within Nineveh and Diyala provinces were part of Iraqi Kurdistan, was approved by 96 of the 111 MPs in the regional parliament in Arbil.

Parliament wants to put the document before Kurdish voters for ratification.

The Iraqi Kurds have long striven to expand their northern territory beyond its current three provinces to other areas where the population was historically Kurdish.

Kurdistan has its own flag which is raised beside the federal flag, and also has its own slogan, national anthem and national day.