ARBIL - Iraq's government cut the autonomous northern Kurdish region's direct air links with the outside world indefinitely on Friday after it voted a massive "yes" in a referendum on independence.

The move effectively isolates and increases the pressure on the Kurds amid soaring regional tensions following Monday's contentious vote.

The central government in Baghdad had ordered the halt to all foreign flights to and from the region from 6:00 pm (1500 GMT).

Foreigners scrambled to fly out of Iraqi Kurdistan from the region's capital Arbil and its second largest city Sulaimaniyah before the ban took effect.

Arbil airport is a key gateway for humanitarian aid workers helping Iraqis affected by the battle against the Islamic State group (IS).

Washington has said it would be willing to facilitate talks between the Iraqi Kurdish authorities and Baghdad to calm escalating tensions over the 92-percent "yes" vote, as a top Shiite cleric called for the crisis to be solved in an Iraqi court.

Neighbouring Turkey and Iran also strongly opposed the vote, fearing it would inflame the separatist aspirations of their own sizeable Kurdish population. Ankara has threatened a series of measures including blocking lifeline oil exports from the region via Turkey.

The Kurds have condemned the flight suspension as "collective punishment".

Iraq's Prime Minister Haider al-Abadi said it was not "punishment" but a legal measure that would be reversed if the transport authority was transferred to Baghdad in line with the Iraqi constitution. The ban has seen people, many of them foreigners, flock to the airport in the regional capital Arbil to avoid being stranded. Iraqi Kurdistan is home to a large international community, most of whom enter on a visa issued by the regional authorities that is not recognised by the central government, so they cannot travel elsewhere in Iraq.

On Friday, around 100 passengers waited eagerly for their planes in Arbil, where the last flight out was to Vienna at 4 pm.

"We were supposed to go back to Brazil next Saturday but we rescheduled our flight because of the border closing," said Isidoro Junior, a 32-year-old volunteer for an NGO providing medical assistance to Iraqis displaced by the war against IS.

"We are a group of 16 people, so it was quite difficult to find enough seats. One of us came here at 2 am to make sure... we would be able to fly out," he said.

At the Turkish Airlines counter, before the last flight left, a one-way ticket to Istanbul cost $743. But it sold for $1,500 online, said one passenger who had paid twice in error, leaving a £3,000 dent in his credit card.

The director of Arbil airport, Talar Faiq Salih said humanitarian, military and diplomatic flights were excluded from the ban.

Earlier, the UN humanitarian office OCHA said it was working to ensure aid could continue to reach tens of thousands of needy Iraqis.

In Sulaimaniyah, foreigners and others needing to leave sped to the airport before the ban took effect, while Kurds who were abroad for business or tourism rushed back from abroad.

"There have been masses of people for two days," said airport spokesman Dana Mohammad Said.

"After 6 pm there will be no more international flights, just internal flights," he said.

The civil aviation authority in Baghdad has said a decision on internal flights will be made later.

Grand Ayatollah Ali al-Sistani, Iraq's highest Shiite religious authority, called for all sides "to abide by the Iraqi constitution and to appeal to High Federal Court to solve the Kurdistan crisis".

"The latest political developments should not have a negative impact on the strong relationship between sons of the homeland, Arabs, Turkmen, Kurds and others," his representative said.

On the international front, the United States said it would be prepared to "help facilitate a conversation" between Arbil and Baghdad.

"We would like to see some calm on all sides," US State Department spokeswoman Heather Nauert said, noting that the US had opposed the referendum "because we thought it would be destabilising".

On Thursday, the spokesman for the international coalition fighting IS in Iraq and neighbouring Syria said the referendum had taken focus away from the war against the jihadists.

But he said there was "absolutely no effect on current military operations out of Arbil using the airport".

Iraqi forces on Friday launched an assault on the northern town of Hawija, one of the last IS bastions in the country along with a stretch of the Euphrates Valley near the border with Syria.

Kurdish forces have been key allies in US-backed offensives against IS in both Syria and Iraq.