Clashes broke out early Monday in northern Iraq as Iraqi forces moved to recapture Kurdish-held oil fields and a military base near the city of Kirkuk, setting the stage for a battle between two U.S. allies.

After a three-day standoff, Iraqi forces advanced into the contested province with the goal of returning to positions they held before 2014, when they fled in the face of an Islamic State push. The positions have since been taken over by Kurdish troops.

The conflict between Kurdi­stan and the Iraqi government over land and oil is decades old, but a Kurdish referendum for independence last month inflamed the tensions. The Iraqi government, as well as the United States, Turkey and Iran all opposed the vote.

The flare-up presents an awkward dilemma for the United States, which has trained and equipped the advancing Iraqi troops, which include elite counterterrorism forces, and the Kurdish peshmerga on the other side.