European nations offered stark warnings Thursday about the waning days of a massive airlift to bring people out of Taliban-controlled Afghanistan, with a British official saying an “imminent attack” could target Kabul’s international airport.

As U.S. President Joe Biden says he’ll stick with his deadline of Aug. 31 to totally withdraw troops from America’s longest war, other nations have landed flight after flight onto the single runway at Kabul’s international airport to help those fleeing.

Now some countries have begun to even pull their soldiers and diplomats out, likely signaling the beginning of the end of one of history’s largest airlifts. The Taliban have, so far, honored a pledge not to attack Western forces during the evacuation, but insist the foreign troops must be out by the deadline.

Overnight, new warnings also emerged from Western capitals about a possible threat from Afghanistan’s Islamic State affiliate, which likely has seen its ranks boosted by the Taliban freeing prisoners across the country. Already, military cargo planes leaving Kabul airport have launched flares to disrupt any potential surface-to-air missile fire as fleeing Afghan troops abandoned heavy weapons and equipment across the country in their collapse following America’s withdrawal of troops.

British Armed Forces Minister James Heappey told the BBC on Thursday there was ”very, very credible reporting of an imminent attack” at the airport, possibly within “hours.”

Heappey conceded that people are desperate to leave and “there is an appetite by many in the queue to take their chances, but the reporting of this threat is very credible indeed and there is a real imminence to it.”

“We will do our best to protect those who are there,” he said. “There is every chance that as further reporting comes in, we may be able to change the advice again and process people anew but there’s no guarantee of that.”

Outside of a missile attack, troops have been worried about the uncontrolled, teeming crowds outside the airport. While the Taliban and others have tried to control them, there’s no formal screening process on the way the airport as there was under Afghanistan’s former government. That means someone carrying a suicide bomb could slip through — or an explosives-laden vehicle could barrel through.

On Wednesday, the U.S. Embassy in Kabul issued a security alert warning American citizens away from three specific airport gates, but gave no further explanation.

Senior U.S. officials said the warning was related to ongoing and specific threats involving the Islamic State and potential vehicle bombs. The officials spoke on condition of anonymity because they were not authorized to discuss ongoing military operations.

The Islamic State affiliate in Afghanistan grew out of disaffected Taliban members who hold an even-more extreme view of Islam, riding on a wave when the militants seized territory across Iraq and Syria. Naming themselves after Khorasan, a historic name for the greater region, the extremists embarked on a series of brutal attacks that included a 2020 assault on a maternity hospital in Kabul that saw infants and women killed.

The Taliban have fought against Islamic State militants in Afghanistan. However, their advance across the country likely saw IS fighters freed alongside the Taliban’s own.

Meanwhile Thursday, French Prime Minister Jean Castex told French radio RTL on that “from tomorrow evening onwards, we are not able to evacuate people from the Kabul airport” due to the upcoming American withdrawal.

Meanwhile, Danish defense minister Trine Bramsen bluntly warned: “It is no longer safe to fly in or out of Kabul.” Denmark’s last flight, carrying 90 people plus soldiers and diplomats, already had left Kabul.

Already, Poland and Belgium have ended their evacuations from Afghanistan.

The Taliban wrested back control of Afghanistan nearly 20 years after they were ousted in a U.S.-led invasion following the 9/11 attacks, which al-Qaida orchestrated while being sheltered by the group. Their return to power has pushed many Afghans to flee, fearing reprisals and a possible return to their brutal rule.