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.
At least 16 Kurdish fighters were killed in the operation, Kurdish Peshmerga commanders said, claiming Iraqi forces used US-supplied weapons against them.
US President Donald Trump insisted Washington would not take sides in the dispute. "We don't like the fact that they're clashing. We're not taking sides," Trump told reporters at the White House.
"We've had for many years a very good relationship with the Kurds as you know and we've also been on the side of Iraq, even though we should have never been there in the first place. We should never have been there. But we're not taking sides in that battle."
The operation puts the US in a dilemma and highlights the complexities of the fight against ISIS -- Washington arms and supports both the Iraqi forces and Kurdish fighters. It also raises the long unanswered questions of how territory might eventually be divided along ethnic lines if ISIS is finally defeated.
The State Department called for calm.
"The United States is very concerned by reports of violence around Kirkuk, Iraq. We are monitoring the situation closely and call on all parties to coordinate military activities and restore calm," spokeswoman Heather Nauert said in a statement overnight.
The US is trying to get its allies in Iraq to stay focused on ISIS, but as the extremist group is now close to defeat in the country, both sides are clearly thinking about where lines will be drawn over former ISIS-held territory.
We strongly urge all parties to avoid provocations that can be exploited by Iraq's enemies who are interested in fueling ethnic and sectarian conflict. In particular, we note that there is still much work to be done to defeat ISIS in Iraq, and continued tensions between Iraqi and Kurdish forces distract from this vital mission. The United States will continue to stand with our Iraqi partners to ensure ISIS's defeat," Nauert said.
The Iraqi operation began on Sunday with forces seizing key assets, including the key Baba Karkar oil and gas field and the K1 military base, Iraqi counterterrorism spokesman Sabah al-Noman told CNN. By Monday, Iraqi forces had seized the entire city and set up checkpoints on its perimeter.
Hundreds of Kurdish families fled Kirkuk on Monday, while video footage from inside Kirkuk showed other residents celebrating with Iraqi flags.
There were also reports of a split between Kurdish factions. The Peshmerga General Command accused members of the Patriotic Union of Kurdistan, a political party within the Kurdistan region, of abandoning their posts as Iraqi forces entered, in what it described as a betrayal.
The Kurdistan region and the greater Kirkuk province have an estimated 15 to 25% of Iraq's oil reserves, with several key oil fields surrounding Kirkuk city. Iraq has one of the world's largest known oil reserves.
Kirkuk was historically a Kurdish-majority Iraqi town, but during his rule, ousted dictator Saddam Hussein moved Arab families in and Kurdish families out to change the area's ethnography, under a policy termed "Arabization." It's also home to Sunni Arabs and Turkmen.
While Baghdad has long said it would not let Kirkuk fall permanently into Kurdish hands, Kurdish fighters have led several key offensives against ISIS. They will likely look for a deal with Baghdad in return for not opposing the Iraqi operation to retake control.
Iraqi forces fled Kirkuk in 2014 as ISIS fighters attempted to secure the territory, shortly after taking over the city of Mosul and establishing their so-called Islamic caliphate across the north of the country. The Kurds sent in their fighters and claimed the city.
The city has suffered a series of major attacks over the past decade from extremists, including al Qaeda in Iraq, targeting mostly security forces there.
After the fall of Saddam, Kurds began returning to Kirkuk, repopulating the city and its surrounding areas.