WASHINGTON (AP) \u2014 The U.S.-led evacuation of Americans and others from the chaotic Kabul airport is accelerating, with a one-day doubling of the number airlifted out of the country, although President Joe Biden is not ruling out extending the mission beyond the Aug. 31 deadline he set before the Taliban's swift takeover.\r\n\r\nOver the 24 hours that ended early morning Monday, 28 U.S. military flights evacuated approximately 10,400 people from Kabul, according to a White House official. In addition, 61 coalition aircraft evacuated approximately 5,900 people. Combined, that is more than double the number airlifted in the previous 24-hour period \u2014 3,900 aboard U.S. military aircraft and 3,900 aboard coalition planes.\r\n\r\nIn 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.\r\n\r\n\u201cThe evacuation of thousands of people from Kabul is going to be hard and painful, no matter when it started, when we began,\u201d Biden said. \u201cIt 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.\u201d\r\n\r\nBiden said military discussions are underway on potentially extending the airlift beyond his Aug. 31 deadline. \u201cOur hope is we will not have to extend, but there are discussions,\u201d he said, suggesting the possibility that the Taliban will be consulted.\r\n\r\nSince Aug. 14, the U.S. has evacuated and facilitated the evacuation of approximately 37,000 people. Since the end of July, we have relocated approximately 42,000 people, the White House official said.\r\n\r\n\u201cWe see no reason why this tempo will not be kept up,\u201d Biden said from the White House. The U.S. military says it has the capacity to fly 5,000 to more than 9,000 people out of Kabul per day.\r\n\r\nBiden 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 \u201csafe zone.\u201d\r\n\r\n\u201cWhat I\u2019m not going to do is talk about the tactical changes we\u2019re making to make sure we maintain as much security as we can,\u201d he said. \u201cWe have constantly, how can I say it, increased rational access to the airport, where more folk can get there more safely. It\u2019s still a dangerous operation but I don\u2019t want to go into the detail of how we\u2019re doing that.\u201d\r\n\r\nLater Biden added: \u201cWe\u2019ve discussed a lot with the Taliban. They\u2019ve been cooperative in extending some of the perimeter.\u201d\r\n\r\nHe said groups of Americans in Kabul are being moved more efficiently and safely to the airport, but he did not provide details.\r\n\r\n\u201cAny American who wants to get home, will get home,\u201d he asserted.\r\n\r\nEarlier Sunday, administration officials said the U.S. military is considering \u201ccreative ways\u201d 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.\r\n\r\nAddressing 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.\r\n\r\nBiden and his top aides have repeatedly cited their concern that extremist groups in Afghanistan will attempt to exploit the chaos around the Kabul airport.\r\n\r\n\u201cThe threat is real, it is acute, it is persistent and something we\u2019re focused with every tool in our arsenal," said Biden's national security adviser, Jake Sullivan.\r\n\r\nThe 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 \u201cseveral thousand.\u201d\r\n\r\nSpeaking on ABC\u2019s \u201cThis Week,\u201d Austin said that as Biden\u2019s Aug. 31 deadline for ending the evacuation operation approaches, he will recommend whether to give it more time.\r\n\r\nRepublicans in Congress stepped up their criticism of Biden's response. \u201cIf 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,\u201d GOP Sen. Joni Ernst of Iowa, an Army veteran, said on ABC's \u201cThis Week.\u201d\r\n\r\nRyan Crocker, who served as U.S. ambassador to Afghanistan under Presidents George W, Bush and Barack Obama, told CBS' \u201cFace the Nation\u201d that Biden\u2019s management of the withdrawal was \u201ccatastrophic\u201d and had unleashed a \u201cglobal crisis.\u201d\r\n\r\nVice 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\u2019s prime minister.\r\n\r\nHarris said that while there \u201cwill be and should be a robust analysis of what has happened,\u201d the current focus \u201chas to be on evacuating American citizens, Afghans who worked with us and vulnerable Afghans, including women and children.\u201d\r\n\r\n\u201cWe 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,\u201d she said.\r\n\r\nSingapore 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 \u201cgrateful\u201d 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.\r\n\r\nA 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.\r\n\r\nIn 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.\r\n\r\n___\r\n\r\nAssociated Press writers Aamer Madhani, Lolita C. Baldor, Ellen Knickmeyer, Hope Yen, Alexandra Jaffe, Jonathan Lemire and Matthew Lee contributed to this report.