Baghdad has said only domestic flights will be permitted from 18:00 (15:00 GMT) unless the Kurds hand over control of Irbil and Sulaimaniya airports.

The Kurdistan Regional Government has said such a ban would be "illegal".

Prime Minister Haider al-Abadi wants the referendum, in which 92% of voters backed secession, to be annulled.

Kurdish leaders insist the referendum was legitimate and that they now have a mandate to start negotiations with Baghdad and neighbouring countries.

Why has Baghdad issued an ultimatum?

Kurds are the fourth-largest ethnic group in the Middle East but they have never obtained a permanent nation state.

In Iraq, where they make up an estimated 15-20% of the population of 37 million, Kurds faced decades of repression before acquiring autonomy in 1991.

On Wednesday, Kurdish officials said 2.8 million people living in the three provinces that form the Kurdistan Region, as well as "areas of Kurdistan outside the region's administration", had voted in favour of independence.

The announcement came despite a last-minute appeal for the result to be "cancelled" from Mr Abadi, who said the referendum was "unconstitutional".

Mr Abadi vowed to "impose Iraq's rule" and reaffirmed his threat to stop direct international flights to and from the Kurdistan Region.

What do the Kurds say?

The Kurdistan Regional Government (KRG) warned on Thursday that any ban would be "completely illegal" and amount to "collective punishment against the Kurds".

Officials said the airports were already subject to the Iraq Civil Aviation Authority and that any restrictions would affect the battle against so-called Islamic State (IS).

Kurdish Peshmerga forces have driven the jihadist group out of large parts of northern Iraq since 2014 with air and ground support from a US-led coalition.

"We have an international community here, so this is not going to be only against Kurdish people," Irbil airport's director, Talar Faiq Salih, told the BBC on Thursday.

He said many refugees used the airport, that it was used by the UN to send humanitarian aid to Syria, and that coalition forces were also hosted there.

"So this airport is meant to be for everything," he said.

However, regional carriers - including Turkish Airlines, EgyptAir and Lebanon's Middle East Airlines - have said they will suspend flights at Baghdad's request.

The KRG also vowed to take legal measures to counter recent decisions by the Iraqi parliament, which on Wednesday asked Mr Abadi to send troops to the oil-rich Kirkuk region and other disputed areas currently controlled by Kurdish forces.

How have Iraq's neighbours reacted?

Turkey and Iran, which have Kurdish minorities and are fiercely opposed to Iraqi Kurds gaining independence, have also increased the pressure on the KRG.

Mr Abadi's office said on Thursday that Turkish Prime Minister Binali Yildirim had agreed to deal exclusively with the central government over crude oil exports. That could result in a key pipeline from the Kirkuk region, which provides the KRG with more than 80% of its income, being cut off.

The Turkish and Iranian militaries have also participated in joint exercises near their borders with the Kurdistan region in recent days.

However, the BBC's Mark Lowen in Irbil says there is no appetite for fresh conflict.

Turkey still depends on the oil pumped from Kirkuk, Western allies are urging restraint and the most likely next step is dialogue, our correspondent adds.

The United States, which was "deeply disappointed" that the referendum was held, has urged both sides to "engage constructively".