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

Speaking at the White House, Biden said 11,000 people had been airlifted 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. Tens of thousands of people remain to join the airlift, which has been slowed by security issues and U.S. bureaucracy hurdles.

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.

At the one-week mark since the Taliban completed its takeover of the country, the U.S.-directed airlift from Kabul continued Sunday even as U.S. officials expressed growing concern about the threat from the Islamic State group.

Biden met with Secretary of State Antony Blinken, Defense Secretary Lloyd Austin and other members of his national security team to discuss the situation in Afghanistan, the White House said. Afghanistan will be the chief topic of discussion when Biden and leaders of the Group of Seven industrialized nations meet virtually Tuesday.

“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 said 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.” He later said this included non-Americans who qualify for evacuation, including Afghans who have applied for Special Immigrant Visas.

Austin noted that the U.S. military on Thursday had used helicopters to move 169 Americans into the airport from the grounds of a nearby hotel in the capital. That is the only announced instance of U.S. forces going beyond the airport to get evacuees.

The British military said Sunday another seven people had been killed in the unceasing crush of crowds outside the airport. The U.S. military took control of the airport for evacuations a week ago as the capital fell to the Taliban. But Taliban forces controlling the streets around the airport, and the throngs of people gathering outside in hope of escape, have made it difficult and dangerous for foreigners and their Afghan allies to get through.