Tensions flare as Iraqi forces near Kurdish-held Kirkuk

Kurds have controlled the city since driving out ISIS in 2014, but it falls outside the internationally recognized autonomous Kurdish region in the northwest of Iraq.

The Iraqi operation is the clearest indication yet of its determination to curtail the territorial ambitions of the Kurds, who included Kirkuk in a disputed independence referendum last month that caused consternation in Baghdad.

By early Monday, Iraqi troops claimed to have taken control of key sites, including the North Gas Company and a power station, according to the Iraqi Joint Operations Command.

Iraqi security forces have said they do not intend to enter the disputed city in the operation, but its position south of the city gives it some control over access, and it is unlikely Baghdad will allow the territory to remain solely in Kurdish hands for long.

The clashes highlight the complexities in the fight against ISIS and the unanswered questions of how territory might eventually be divided along ethnic lines.

What's happened?

The Kurdistan Regional Security Council (KRSC) said Kurdish fighters, known as the Peshmerga, were attacked using "US military equipment, including Abrams tanks and Humvees." It added the Peshmerga had destroyed a number of those Humvees.

The US-led coalition against ISIS said it was closely monitoring the situation and is urging all sides to "avoid escalatory actions." Earlier, the Pentagon echoed this stance. Both the Iraqi military and the Kurdish Peshmerga have been trained, funded and equipped by the US for their participation in the coalition fighting ISIS.

The Kurdish regional security council tweeted on Sunday that Iraqi forces and the PMU were intending to take over a military base and major oil fields. The province has one of the biggest oil fields in the country, more than 6% of the world's oil comes from the Kirkuk region.

A long contentious city

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 northwest of the country.

The Kurds, however, sent in their fighters and claimed the city. It's been in their hands ever since, along with other disputed areas they took over when their fighters routed the Islamic State.

Baghdad wants the oil-rich city back, but the Kurds are refusing to return it. Senior members of the Iraqi Kurdish leadership have called Kirkuk a Kurdish "Jerusalem" for decades," said CNN's Ivan Watson, who has long reported from the region.

"When the US military invaded Iraq in 2003, it was Kurdish Peshmerga militias who first stormed Kirkuk, ripping down statues of Saddam Hussein and quickly laying claim to the city. The Kurds have worked hard ever since to maintain control there," he said.

On Twitter Monday, Iraqi Prime Minister Haider al-Abadi called on citizens in Kirkuk to cooperate with Iraqi security forces, who he has directed to enter the city to impose security and provide protection for Kirkuk's residents.

Days earlier, he sought to downplay talk of impending clashes between Iraqi and Kurdish forces in the city. "Our armed forces cannot and will not attack our citizens, whether Arab or Kurd. The fake news being spread has a deplorable agenda behind it," he tweeted on Friday.

The Iraqi Joint Operations Command warned armed groups in Kirkuk against firing at Iraqi forces, Iraqi state TV reported.

Orders not to 'initiate war'

According to a spokesman for Masoud Barzani, the president of the Kurdish Regional Government, the Peshmerga were ordered not to "initiate any war, but if any advancing militia starts shooting, then Peshmerga have been given a green light to use every power to stand against them."

Senior adviser Hemin Hawrami tweeted early Monday that "thousands of volunteers from other cities of Kurdistan are pouring in to Kirkuk." He accused al-Abadi of starting a war by using the Shiite militia units on Kirkuk. "He should wait, Kurdistan reaction will be much stronger than they expected."

Units of Iraqi forces have moved from the villages of al-Basheer and Taza and are approaching the city of Kirkuk, but no forces have entered the city so far, said Mahmoud Haji Mahmoud, commander of the Peshmerga forces in western Kirkuk.

He said advancing Iraqi troops are within one kilometer (0.62 miles) of the perimeter of the city and are setting up berms to use bulldozers near Peshmerga positions.

Three soldiers within the advanced Iraqi Counter Terrorism Forces told CNN that the Iraqi units been given orders to advance towards the south and west of the city's perimeter, secure entrances and take over a number of vital facilities located in that section.

Kurds seek independence

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.

The city had 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 Hussein, Kurds began returning to Kirkuk, repopulating the city and its surrounding areas in the event of an eventual referendum on whether the city should be part of a future Kurdistan or remain in Iraq.

The Kurds last month voted overwhelmingly for independence from Iraq. In response, Baghdad shut down overseas flights to the KRG's international airports. The Kurds are holding firm however, and the status of Kirkuk is as much in doubt as ever.