The United States has made an urgent call for calm in northern Iraq as Kurdish fighters and Iraqi forces -- two of Washington's key allies in the region -- clash over disputed territory.

Iraqi forces seized the coveted oil-rich city of Kirkuk on Monday after three years under Kurdish control. The Kurds took control of the city after it was abandoned by Iraqi government forces during ISIS' lightning offensive in 2014.

But Iraqi Prime Minsiter Haider al-Abadi made clear his forces would eventually come back for it and he ordered the operation to "secure" it on Sunday, weeks after the Kurds held an independence referendum claiming the disputed city as their own.