The U.S. Embassy warned Americans to stay away from the Kabul airport and told anyone outside the perimeter to “leave immediately,” citing unnamed security threats.

The British and Australian governments issued similar warnings, with Australian officials describing “an ongoing and very high threat of terrorist attack.”

The warnings came as the last of the estimated 1,500 Americans and countless other foreigners still in Afghanistan try to make it to the airport to leave before the U.S. withdrawal on Aug. 31. Thousands of Afghan nationals are camped outside the perimeter of the airport in desperate attempts to escape on the last flights out, some with documents allowing them to leave.

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A senior U.S. official, who spoke on the condition of anonymity about confidential assessments, confirmed that the United States was tracking a “specific” and “credible” threat at the airport from the Islamic State affiliate in Afghanistan, which has carried out dozens of attacks in recent years, many targeting ethnic minorities and other civilians.

The U.S. Embassy in Kabul cited three areas of particular concern in its advisory.

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“U.S. citizens who are at the Abbey Gate, East Gate, or North Gate now should leave immediately,” the statement said, without further detail.

Marise Payne, Australia’s foreign minister, said at a news conference Thursday that the Taliban will allow Australian citizens and visa holders to leave safely but added, “Our travel advice remains: You should not come to Hamid Karzai airport because it is not safe to do so, and if you are in Kabul, you should shelter in place, move to a safe location and await further advice.”

The U.S. government has been warning about potential security threats at the airport, and access to the airport has been adjusted accordingly, with some gates temporarily closed.