WASHINGTON (AP) — The United States says as many as 1,500 Americans may be awaiting evacuation from Afghanistan amid growing warnings Thursday of terrorist threats targeting the Kabul airport as President Joe Biden's deadline for withdrawing troops fast approaches.

Untold thousands of at-risk Afghans are struggling to get into the airport even as many thousands of other Afghans already had been flown to safety in nearly two weeks of round-the-clock flights.

The airlift continued Thursday despite warnings of vehicle-borne bomb threats near the airport. The White House said 13,400 people had been evacuated in the 24 hours that ended early Thursday morning Washington time. That included 5,100 people aboard U.S. military planes and 8,300 on coalition and partner aircraft. That was a substantial drop from the 19,000 airlifted by all means the day before.

Several of the Americans working phones and trying to pull strings to get out former Afghan colleagues, women's advocates, journalists and other vulnerable Afghans said they were still waiting for U.S. action.

“It’s 100% up to the Afghans to take these risks and try to fight their way out,” said Sunil Varghese, policy director with the International Refugee Assistance Project.

Secretary of State Antony Blinken emphasized at a State Department briefing on Wednesday that “ evacuating Americans is our top priority. ”

He added: “We’re also committed to getting out as many Afghans at-risk as we can before the 31st," when Biden plans to pull out the last of thousands of American troops.

As more nations began shutting down own evacuation flights and pulling out before the U.S. withdrawal, there were new European warnings about the threats. British Armed Forces Minister James Heappey told the BBC that there was ”very, very credible reporting of an imminent attack” at the airport, possibly within “hours.”

The U.S. Embassy in Kabul, the capital, issued a security alert Wednesday warning American citizens away from three specific airport gates. Senior U.S. officials said the warning was related to ongoing and specific threats involving the Islamic State and potential vehicle bombs, which have set U.S. officials on edge in the final days of the American drawdown. The officials insisted on anonymity because they were not authorized to discuss ongoing military operations.

Blinken said the State Department estimates there were about 6,000 Americans wanting to leave Afghanistan when the airlift began Aug. 14, as the Taliban took the capital after a stunning military conquest. About 4,500 Americans have been evacuated so far, Blinken said, and among the rest “some are understandably very scared.”

The 6,000 figure is the first firm estimate by the State Department of how many Americans were seeking to get out. U.S. officials early in the evacuation estimated as many as 15,000, including dual citizens, lived in Afghanistan. The figure does not include U.S. Green Card holders.

About 500 Americans have been contacted with instructions on when and how to get to the chaotic Kabul airport to catch evacuation flights.

In addition, 1,000 or perhaps fewer are being contacted to determine whether they still want to leave. Blinken said some of these may already have left the country, some may want to remain and some may not actually be American citizens.

Blinken emphasized that the U.S. and other governments plan to continue assisting Afghans and Americans who want to leave after the deadline for Biden's planned end to the evacuation and the two-decade U.S. military role in Afghanistan.

Biden has cited what he U.S. says are rising security threats to U.S. forces, including from an affiliate of the Islamic State terror group, for his determination to stick with Tuesday's withdrawal deadline.

The U.S. Embassy has already been evacuated; staff are operating from the Kabul airport and the last are to leave by Tuesday.

Biden said this week he had asked his national security team for contingency plans in case he decides to extend the deadline. Taliban leaders who took control of Afghanistan this month say they will not tolerate any extensions to the Tuesday deadline. But Taliban spokesman Suhail Shaheen tweeted that “people with legal documents” will still be able to fly out via commercial flights after Tuesday.

U.S. troops are anchoring a multinational evacuation from the airport. The White House says the airlift overall has flown out 82,300 Afghans, Americans and others on a mix of U.S., international and private flights.

The withdrawal comes under a 2020 deal negotiated by President Donald Trump with the Taliban.

Refugee groups are describing a different picture than the Biden administration is when it comes to many Afghans: a disorganized, barely-there U.S. evacuation effort that leaves the most desperate to risk beatings and death at Taliban checkpoints. Some Afghans are reported being turned away from the Kabul airport by American forces controlling the gates, despite having approval for flights.

U.S. military and diplomatic officials appear to still be compiling lists of eligible Afghans but have yet to disclose how many may be evacuated — and how — private Americans and American organizations said.

“We still have 1,200 Afghans with visas that are outside the airport and haven’t got in,” said James Miervaldis with No One Left Behind, one of dozens of veterans groups working to get out Afghans who worked with the U.S. military during America’s nearly 20 years of combat in the country.. “We’re waiting to hear from the US. government and haven’t heard yet.”

U.S.-based organizations, speaking on background to discuss sensitive matters, cite accounts from witnesses on the ground as saying some American citizens, and family members of Afghans with green cards, still were having trouble pushing and talking their way into the Kabul airport for flights.

Pentagon spokesman John Kirby said the U.S. military will preserve as much airlift capacity at the airport as possible in the coming days. But he added that in the final days and hours there will have to be a balance in getting out U.S. troops and their equipment as well as evacuees.

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Associated Press writers Lolita C. Baldor in Washington and James LaPorta in Boca Raton, Florida, contributed to this report.