An Iraqi government order that international airlines halt all flights in and out of the cities of Erbil and Sulaimaniya in the region administered by the Kurdistan Regional Government (KRG) is set to kick in on Friday.

The decision to shut down the flights comes amid tensions over an overwhelming "Yes" vote in an independence referendum held this week in Iraq's Kurdish region and disputed territories.

Iraq's Transport Ministry ordered international airlines to halt service to Erbil, the capital of the KRG, and Sulaimaniya, its second city. Regional airlines have said they will honour the flight ban.

The non-binding referendum - in which the Kurds voted overwhelmingly in favour of independence from Iraq - was billed by Kurdish leaders as an exercise in self-determination. The idea of an independent state has been central to Kurdish politics for decades.

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All international flights to and from Iraq's Kurdish region were set to end at 6pm (15:00 GMT).

Almost all foreign airlines suspended flights to the airports of Erbil and Sulaimaniya in compliance with the notice from the government in Baghdad, which has control over the country's airspace.

Lufthansa and Austrian were the only carriers to have kept at least one flight still scheduled after the deadline. But a spokesman for Lufthansa Group said on Friday it was checking whether it would have to cancel flights.

Domestic flights are still permitted to and from the KRG, so travellers are expected to get there mostly by transiting via Baghdad's airport, which will come under strain from the extra traffic.

Kurdish airports handle 40-50 percent of Iraq's total international traffic, Taher Abdallah, Sulaimaniya airport's director-general, told reporters on Wednesday.

Despite the threatened flight ban, anti-ISIL coalition military air operations from Erbil airport continue as normal, US-led coalition spokesman Colonel Ryan Dillon told reporters at the Pentagon on Thursday from his headquarters in Baghdad.

More broadly, Dillon said the fallout from the Kurdish referendum had diminished the focus on fighting the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group.

"What I'll say now is that there is a lot of posturing and a lot of things that have been said about what could or may happen," he said. "The focus, which used to be like a laser beam on [ISIL], is now not 100 percent there."

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Dillon said Peshmerga and US military planners and advisers also had lost some of their focus as a result of the referendum. US military planners have had to spend the time to "play out the what-ifs" that arise from the political and military implications, he said.

Baghdad announced on Thursday that Turkey - an indispensable trade partner to the region and once a key political ally - will now only deal with Iraq's central government on oil sales. This could deprive the Kurdish region of more than 80 percent of its income.

Ankara had forged close ties to Iraq's Kurdish region but strongly opposes its moves towards independence, fearing it could inspire Turkey's Kurdish minority.