WASHINGTON (AP) — President Joe Biden said Sunday the U.S.-led evacuation of Americans, at-risk Afghans and others from the Kabul airport accelerated this weekend, although it remains vulnerable to threats posed by the Islamic State extremist group.

One week after the Taliban completed its takeover of Afghanistan by capturing Kabul, Biden said discussions are underway among military officials about potentially extending the airlift beyond Biden's Aug. 31 deadline. “Our hope is we will not have to extend, but there are discussions,” he said, suggesting the possibility that the Taliban will be consulted.

Since Aug. 14, one day before the Taliban entered Kabul, the airlift has evacuated 28,000 people, Biden said. He said that included 11,000 who had departed from Kabul in a 36-hour period this weekend, but he did not provide details. The number appeared to include flights by charter and non-U.S. military aircraft as well as the U.S. Air Force C-17 and C-130 transport planes that have been flying daily from the capital. The U.S. military is controlling air traffic on both the civilian and military sides of the airport.

Tens of thousands of people remain to join the airlift, which has been slowed by security issues and U.S. bureaucracy hurdles.

Biden asserted, without a full explanation, that U.S. forces have managed to improve access to the airport for Americans and others seeking to get on flights. He suggested that the perimeter had been extended, widening a “safe zone.”

“What I’m not going to do is talk about the tactical changes we’re making to make sure we maintain as much security as we can," he said. "We have constantly, how can I say it, increased rational access to the airport, where more folk can get there more safely. It's still a dangerous operation but I don’t want to go into the detail of how we’re doing that.”

Later Biden added: “We've discussed a lot with the Taliban. They’ve been cooperative in extending some of the perimeter.”

He said groups of Americans in Kabul are being moved more efficiently and safely to the airport, but he provided no details.

“Any American who wants to get home, will get home,” he asserted.

Earlier Sunday, administration officials said the U.S. military is considering “creative ways” to get Americans and others into the Kabul airport for evacuation from Afghanistan amid “acute” security threats, and the Pentagon on Sunday ordered six U.S. commercial airlines to help move evacuees from temporary sites outside of Afghanistan.

Addressing a criticism cited by many Republicans, Biden said no Afghan evacuees are being flown directly to the United States from Afghanistan without prior screening. He said they are being screened in third countries.

Biden and his top aides have repeatedly cited their concern that extremist groups in Afghanistan will attempt to exploit the chaos around the Kabul airport.

“The threat is real, it is acute, it is persistent and something we’re focused with every tool in our arsenal," said Biden's national security adviser, Jake Sullivan.

Sullivan said on CNN's “State of the Union” that 3,900 people had been airlifted out of Kabul on U.S. military flights over the past 24 hours. A U.S. defense official, speaking on condition of anonymity to provide details not yet made public, said those people were flown on a total of 23 flights — 14 by C-17 transports and nine aboard C-130 cargo planes.

That represents an increase from 1,600 flown out aboard U.S. military planes in the previous 24 hours, but remains far below the 5,000 to 9,000 that the military says it has the capacity to airlift daily. Sullivan also said about 3,900 people were airlifted on non-U.S. military flights over the past 24 hours.

The Biden administration has given no firm estimate of the number of Americans seeking to leave Afghanistan. Some have put the total between 10,000 and 15.000. Sullivan on Sunday put it at “several thousand.”

Speaking on ABC’s “This Week,” Austin said that as Biden’s Aug. 31 deadline for ending the evacuation operation approaches, he will recommend whether to give it more time. Tens of thousands of Americans and others have yet to be flown out of the country.

Austin’s interview with ABC aired Sunday but was taped Saturday. In a notice Sunday, the State Department urged people seeking to leave Afghanistan as part of an organized private evacuation effort not come to the Kabul airport “until you have received specific instructions” to do so from the U.S. Embassy’s flight organizer. The notice said that others, including American citizens, who have received specific instructions from the embassy to make their way to the airport should do so.

Austin said the airlift would continue for as long as possible.

“We’re gonna try our very best to get everybody, every American citizen who wants to get out, out,” Austin said in the interview. “And we’ve got -- we continue to look at different ways to -- in creative ways -- to reach out and contact American citizens and help them get into the airfield.”

The British military said Sunday another seven people had been killed in the unceasing crush of crowds outside the airport.

Republicans in Congress stepped up their criticism of Biden's response. “If the Taliban is saying that Americans can travel safely to the airport, then there is no better way to make sure they get safely to the airport than to use our military to escort them,” GOP Sen. Joni Ernst of Iowa, an Army veteran, said on ABC's “This Week.”

Ryan Crocker, who served as U.S. ambassador to Afghanistan under Presidents George W, Bush and Barack Obama, told CBS' “Face the Nation” that Biden’s management of the withdrawal was “catastrophic” and had unleashed a “global crisis.”

A central problem in the evacuation operation is processing evacuees once they reach other countries in the region and in Europe. Those temporary waystations, including in Qatar, Bahrain and Germany, are sometimes reaching capacity, although new sites are being made available, including in Spain.

In an attempt to alleviate that, and to free up military aircraft for missions from Kabul, the Pentagon on Sunday activated the Civil Reserve Air Fleet. The Defense Department said 18 aircraft from American Airlines, Atlas Air, Delta Air Lines, Omni Air, Hawaiian Airlines and United Airlines will be directed to ferry evacuees from interim waystations. The airlines will not fly into Afghanistan. The six participating airlines have agreed to assist for a little less than two weeks, which roughly coincides with the currently planned duration of the airlift, which is to end Aug. 31.

The civil airline reserve system was last activated in 2003 for the Iraq War. The commercial airliners will retain their civilian status but the military's Air Mobility Command will control the flights.

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Associated Press writers Aamer Madhani, Lolita C. Baldor, Ellen Knickmeyer, Hope Yen and Matthew Lee contributed to this report.