IRBIL, Iraq: Iraq’s military was preparing to take control of the international borders of the northern Kurdish region Friday, as a flight ban halted all international flights from servicing the territory’s airports and the U.S. said it did not recognize the Kurdish independence referendum. Iraqi troops now in Turkey and Iran would start Saturday morning to enforce control over the border crossings out of the Kurdish region, Iraqi officials told the Associated Press.

They will not enter the Kurdish region, but instead Iraqi customs administrators backed by the troops will set up control points just outside the Kurdish border stations, the officials said. That way all traffic passing back and forth would have to go through them.

The step will be the first movement of troops – outside of joint military exercises held by Turkey, Iran and Iraq – in response to this week’s referendum in which Kurds voted by more than 90 percent to back independence from Iraq for their self-rule zone and other areas they have captured in the past year.

The escalation feeds worries in the U.S., a close ally of both the Kurds and Baghdad, that the referendum vote could lead to violence, setting off an unpredictable chain of events.

U.S. Secretary of State Rex Tillerson said Friday that the United States did not recognize the Kurdistan Regional Government’s unilateral independence referendum in Iraqi Kurdistan.

“The vote and the results lack legitimacy and we continue to support a united, federal, democratic and prosperous Iraq,” he said, urging “calm and an end to vocal recriminations and threats of reciprocal actions.”

The nonbinding referendum will not immediately result in independence. But Kurdish leaders have said they will use it to press for negotiations on eventually forming their own state. Consequently, Iraq, Iran and Turkey have moved to isolate the region. Iran announced Friday a ban on oil imports and exports with the Iraqi Kurdish region, the state news agency announced.

At Irbil’s international airport, hundreds of passengers lined up for flights out of the Kurdish region in the hours before the central government’s flight ban took effect Friday evening. Baghdad had demanded the region hand over the airport to its authority or else face a ban. Talar Saleh, the airport’s general director, said Kurdish authorities had attempted to meet with officials from the central government to comply with the demand. But “so far, up to this moment, there is no reply from Baghdad,” she said at a news conference held at the airport. While Baghdad controls the airspace over the Kurdish region, immigration and security inside the airports are controlled by local Kurdish region officials and security forces.

Many of the hundreds of people traveling Friday afternoon were foreigners ordered to leave the region by the companies they work for.

Iraqi Prime Minister Haider al-Abadi said the flight ban was not intended to hold the Kurdish region captive. “Central government control of air and land ports in the Kurdistan region is not meant to starve, besiege and prevent [the delivery of] supplies to the citizens in the region as alleged by some Kurdistan region officials,” he said in a statement from his office.

At Friday prayers from the holy city of Karbala, Iraq’s top Shiite Islamic scholar Grand Ayatollah Ali Sistani warned the Kurdish vote will have consequences on “our dear Kurdish citizens.”