Turkey Monday denied media reports it had closed its southern border gate with northern Iraq in response to the independence referendum in Iraqi Kurdistan, but said strict controls were in place.

"Habur border gate has not been closed," Customs Minister Bulent Tufenkci was quoted as saying by the state-run Anadolu news agency. "A tight control is in place for security reasons."

Witnesses in the region also said the border gate remained open but cross-border traffic was slow.

Iraqi Kurds were bracing for a non-binding vote on independence set for Monday, despite warnings from Baghdad as well as neighbouring countries Iran and Turkey. 

Ankara on Monday slammed the referendum bid by the Kurdistan Regional Government (KRG) as "null and void" and said it would not recognise it.

"We do not recognise this initiative which is deficient of any legal basis and lacks legitimacy," the Turkish foreign ministry said in a statement. 

Ankara also vowed to take every measure in case any radicals sought to exploit the circumstances that will emerge following the referendum, or any attempt to carry out acts that target "our national security".

The ministry also updated its travel advice for Iraqi Kurdistan, strongly advising Turks to leave the cities of Dohuk, Arbil and Sulaymaniyah if they did not need to stay because of the security risks related to the poll.

Despite enjoying strong ties with the KRG in recent years, Turkey fears the vote could stoke separatist aspirations among its own sizeable Kurdish minority.

Turkey has warned of reprisals if the referendum goes ahead as planned but has never made clear what concrete action it might take against the regional government of Massud Barzani.

Business circles warn the closure of the Habur border gate could undermine $7 billion trade between Ankara and Arbil.