The US has called for “calm” after Iraqi government forces seized the northern city of Kirkuk and key installations from Kurdish control.
State department spokeswoman Heather Nauert urged all parties to “avoid further clashes”.
Iraqi soldiers moved into Kirkuk three weeks after the Kurdistan region held a controversial independence referendum.
They are aiming to retake areas under Kurdish control since Islamic State militants swept through the region.
Residents of Kurdish-controlled areas, including Kirkuk, overwhelmingly backed secession from Iraq in a vote on 25 September.
While Kirkuk is outside Iraqi Kurdistan, Kurdish voters in the city were allowed to take part.
Iraqi Prime Minister Haider Al-Abadi had denounced the vote as unconstitutional. But the Kurdistan regional government insisted it was legitimate.
Meanwhile, reports say a militia backed by the Baghdad government has taken control of the town of Sinjar in the north-western Nineveh province.
The town is claimed by both Kurdish and central Iraqi authorities. The transfer happened without violence after Kurdish peshmerga soldiers withdrew from the area, the reports added.