WASHINGTON (NewsNation Now) — 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 security issues and U.S. bureaucracy hurdles have slowed.

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. That worry comes on top of obstacles to that mission from the Taliban, as well as U.S. government bureaucratic problems.

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., and planned to provide a public update on Afghanistan later in the day. Afghanistan will be the chief topic of discussion when Biden and leaders of the Group of Seven industrialized nations meet virtually on 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 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.” He later said this included non-Americans who qualify for evacuation, including Afghans who have applied for Special Immigrant Visas.

A NATO official said that at least 20 people have died in the past seven days in and around the airport. Some were shot and others died in stampedes, witnesses have said.

Panicked Afghans have tried to get on flights abroad, fearing reprisals and a return to a harsh version of Islamic law the Sunni Muslim group exercised when it was in power two decades ago.

Pentagon Press Secretary John Kirby said the department does not anticipate a major impact on commercial flights from this activation of commercial airlines.

According to Kirby, those aircraft will not fly into Hamid Karzai International Airport in Kabul. Instead, they will be used to move passengers from way stations once they leave Kabul, allowing the U.S. military to focus on the Afghanistan portion of the evacuation.

This would be only the third time the “Civil Reserve Air Fleet” has been activated. The first time was during the Gulf War in 1990 and then during the invasion of Iraq in 2002.

The first occurred in support of Operations Desert Shield/Storm (Aug. 1990 to May 1991), and the second was for Operation Iraqi Freedom (Feb. 2002 to June 2003).

Army Major General William Taylor told a Pentagon briefing on Saturday that 5,800 U.S. troops remain at the airport and that the facility “remains secure.” Taylor said some gates into the airport were temporarily closed and reopened over the past day to facilitate a safe influx of evacuees.

Taylor said that the United States has evacuated 17,000 people, including 2,500 Americans, from Kabul in the past week.

U.S. National Security Adviser Jake Sullivan told CNN that 3,900 people had been airlifted out of Kabul on U.S. military flights in the past 24 hours, up from 1,600 the previous day. That’s in addition to about 3,900 people airlifted on non-U.S. military flights over the past 24 hours. However, it remains far below the 5,000 to 9,000 that the military says it has the capacity to airlift daily.

Britain said it had airlifted more than 5,000 people, including 1,000, in the last 14 hours.

The Taliban swoop into power, and the collapse of the Afghan army came as U.S.-led forces were withdrawing after a 20-year war that Biden sought to conclude.

On Saturday, former President Donald Trump called it “the greatest foreign policy humiliation” in U.S. history, even though his own administration negotiated the withdrawal deal last year.

But authority over America’s longest war finally fell into Biden’s hands this year and he insisted that the U.S. withdraw from Afghanistan, settling on an Aug. 31 deadline.

After months of largely focusing on quelling the pandemic and stimulating the economy, the chaos in Afghanistan triggered the first foreign policy crisis of Biden’s presidency, temporarily drowning out his other priorities.

The Associated Press and Reuters contributed to this report.