The Iraqi government has accused Kurdish authorities of bringing fighters from Turkey's outlawed Kurdistan Workers Party (PKK) to the disputed province of Kirkuk, in a move it called a "declaration of war".

The accusation was quickly rejected by Kurdish officials later on Sunday.

Thousands of Iraqi soldiers and allied militia are locked in a tense armed standoff with Kurdish forces in the oil-rich province amid a sharp dispute between the central government in Baghdad and the autonomous Kurdish region of Iraq.

The National Security Council headed by Iraqi Prime Minister Haider al-Abadi said it viewed as a "dangerous escalation" the presence of armed men "not belonging to the regular security forces in Kirkuk", including PKK fighters.

"It is impossible to remain silent" faced with "a declaration of war towards Iraqis and government forces", the council said a statement posted on the prime minister's official Twitter account.

"The central government and regular forces will carry out their duty of defending the Iraqi people in all its components including the Kurds, and of defending Iraq's sovereignty and unity," it added.

Vahal Ali, a media assistant to Kurdistan Regional Government (KRG) President Masoud Barzani, denied that any PKK forces were present in Kirkuk.

"This is false, there are no PKK in Kirkuk, only Peshmerga," he told Reuters news agency, referring to KRG military forces.

General Jabar Yawer, secretary-general of the Peshmerga ministry, told AFP news that "there are no PKK forces in Kirkuk, but there are some volunteers who sympathise with the PKK".

There were also "other volunteers, independents and Islamists fighting Daesh since 2014", he added, using an Arabic acronym for the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) armed group.

"They support the Peshmerga. They are irregular forces," Yawer said.

Rising tensions

The two sides have been at loggerheads since the Kurds voted overwhelmingly for secession in a September 25 referendum that Baghdad declared illegal.

On Sunday, Kurdish leaders rejected a demand by Baghdad to cancel the outcome of the referendum as a precondition for talks to resolve the dispute.

Barzani and other Kurdish leaders, who held talks in the town of Dokan, called for negotiations without preconditions, renewing an offer to "resolve peacefully" the crisis with Baghdad.

"The outcome of the referendum will not be nullified," the Kurdish region's Prime Minister Nechirvan Barzani said following a joint meeting of the Kurdistan Democratic Party (KDP) and the Patriotic Union of Kurdistan (PUK) - the region's two main parties.

"Talk of cancelling these results is out of the question and will not address the problems," he added at a press conference.

The Kurdish leaders rejected what they described as "military threats" from Iraqi forces against Peshmerga fighters, and pledged to defend Kurdish-held territory in the event of an attack.