WASHINGTON (AP) — President Joe Biden says the “hard and painful” airlift of Americans and tens of thousands of others from Afghanistan's capital is accelerating, but he would not rule out extending it beyond the Aug. 31 deadline he set before the Taliban's swift takeover.

In remarks at the White House on Sunday, one week after the Taliban completed their victory by capturing Kabul, Biden defended his decision to end the war and insisted that getting all Americans out of the country would have been difficult in the best of circumstances. Critics have blasted Biden for a grave error in judgment by waiting too long to begin organizing an evacuation, which became captive to the fear and panic set off by the government's sudden collapse.

“The evacuation of thousands of people from Kabul is going to be hard and painful, no matter when it started, when we began,” Biden said. “It would have been true if we'd started a month ago, or a month from now. There is no way to evacuate this many people without pain and loss of heartbreaking images you see on television.”

Biden said military discussions are underway on 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.

In a statement later Sunday, a White House official said eight U.S. military flights — seven C-17s and one C-130 — evacuated about 1,700 passengers from Hamid Karzai International Airport in a 12-hour period ending at 3 p.m. EDT. In addition, 39 coalition aircraft took off with approximately 3,400 passengers, the official said.

Since Aug. 14, the U.S. has evacuated and facilitated the evacuation of about 30,300 people on military and coalition flights, the official said. Tens of thousands of people remain to join the airlift, which has been slowed by security issues and U.S. bureaucracy hurdles.

On Saturday, 3,900 Americans were flown out of Kabul on 23 U.S. military flights, Biden said.

“We see no reason why this tempo will not be kept up,” he said. The U.S. military says it has the capacity to fly 5,000 to 9,000 people out of Kabul per day.

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 did not provide 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, 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.

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.

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.”

Vice President Kamala Harris got plenty of reminders of the Afghanistan situation during her visit to Singapore. She dismissed multiple questions over what went wrong there during a press conference in Singapore after her meeting with the nation’s prime minister.

Harris said that while there “will be and should be a robust analysis of what has happened,” the current focus “has to be on evacuating American citizens, Afghans who worked with us and vulnerable Afghans, including women and children.”

“We cannot be in any way distracted from what must be our primary mission right now, which is evacuating people from that region who deserve to be evacuated,” she said.

Singapore Prime Minister Lee Hsien Loong offered support for the U.S. decision to withdraw from Afghanistan, saying the country understands the decision and that they were “grateful” for the U.S. efforts to combat terrorism in the region. He also announced that Singapore would offer use of its Air Force planes to help with the evacuation there.

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.

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