Monday's Peshmerga statement accused a group within the PUK of "treason" for assisting Baghdad's advance. So far there has been little shooting as the Peshmerga abandoned their positions in what appears to have been a prearranged withdrawal.
Oilfields near Kirkuk halted production, but Baghdad said it would be quickly restarted. He feared the referendum, guaranteed to alienate all the Kurds'allies, would turn out to a political error with similar calamitous consequences.
At the same time, the US Department of Defense refused tcomment on reports speculating that Iraqi Security Forces were using US military equipment and training to push Kurdish Pershmerga fighters out of areas surrounding Kirkuk.
The Iraqi National Security Council calls this a "declaration of war towards Iraqis and government forces".
The elite, US -trained "Counter Terrorism Service, the 9th armored division and Federal Police have taken control of vast areas of Kirkuk without confrontations", it said, adding that oilfields and Kurdish military positions were captured.
The central government controlled Kirkuk until 2014, when the Iraqi army crumbled as the "Islamic State" (IS) swept through large parts of Iraq and Syria. But it was adamantly opposed by the Iraqi central government, Iran, Turkey as well as traditional Kurdish allies such as the U.S. and Europeans, leaving Mr Barzani isolated in the face of superior forces.
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"This is false, there are no PKK in Kirkuk, only Peshmerga", he told Reuters, referring to KRG military forces.
Oil prices shot up as a result of the military action, with Brent crude gaining as much as 1.8% to $58.22 per barrel by Monday afternoon.
The referendum had faced strong opposition from most regional and worldwide actors (including the U.S., Turkey and Iran), who warned that the poll would distract from Iraq's fight against terrorism and further destabilize the already-volatile region.
The Kurdish secession bid was strongly opposed by neighbours Iran and Turkey.
As well as heavily armed federal troops, members of the Hashed al-Shaabi, or Popular Mobilisation forces, which are dominated by Iran-backed Shiite militias, have massed around Kirkuk.
Swiftly. Iraq's army, its anti-terrorism forces and the federal police began their operations before dawn Monday.
Oil-rich Kirkuk is claimed by both the Kurdish Regional Government and Baghdad.
Kurdish forces have been in control of six fields in the Kirkuk region providing some 340,000 of the 550,000 barrels per day exported by the regional administration.
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"We have no orders to enter the city, just to secure the surroundings", a military commander involved in the operation said, adding that the Kurdish forces had pulled out in an orderly manner from the position taken by the Iraqi forces. We have an agreement with some Kurdish leaders that the oil and gas facilities should stay out of the conflict, the ministry official said.
The meeting was attended by Mr Barzani, Iraqi President Fuad Masum and Ms Hero Talabani, the wife of Mr Jalal Talabani, a Kurdish leader who died earlier this month.
Both parties control their own Peshmerga units.
The PUK had supported a UN-backed plan for negotiations with Baghdad in exchange for dropping the referendum.
"It's comical, really", said a Kurdish official with the KDP, talking about US silence given the presence of Iran-supported militias in the advance.
A Kurdish health official said at least 10 peshmerga fighters were killed and 27 wounded during fighting overnight, but there was no confirmation of the toll from the Kurdish government.
Iraqi and Kurdish Peshmerga forces early yesterday exchanged artillery fire south of the capital of the province, after the launch of the operation on Sunday night.
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