Activating a plan used only twice before, the Biden administration on Sunday ordered the use of commercial aircraft to help ferry people evacuated from Afghanistan.

Chaotic scenes continue at the airport in Kabul, a week after the city fell to the Taliban. On Saturday, the US advised Americans in the city not to travel to the airport. Officials told US outlets there were concerns that Islamic State militants might mount an attack.

At the White House, Joe Biden was due to speak to the nation on Sunday afternoon. The press secretary, Jen Psaki, said the president would meet virtually with G7 leaders on Tuesday.

The activation of the Civil Reserve Air Fleet, for only the third time in history, was ordered by Lloyd Austin, the defense secretary.

A Pentagon spokesman said the aircraft would not fly into Kabul but would be used to transport people already flown out. The administration asked for three planes each from American Airlines, Atlas Air, Delta Air Lines and Omni Air; two from Hawaiian Airlines; and four from United Airlines.

Speaking to ABC’s This Week, Austin said: “We’re gonna try our very best to get everybody, every American citizen who wants to get out, out. And we continue to look at different ways – creative ways – to reach out and contact American citizens and help them get into the airfield.”

Asked about plans to continue to evacuate Afghan citizens who have worked with US troops, diplomats, media and non-governmental groups since 2001, and are now seeking to escape likely Taliban retribution, Austin said: “The people that are in the Special Immigrant Visa program are very, very important to us … we want to evacuate them as well.”

The secretary of state, Tony Blinken, told CBS’s Face the Nation: “We’ve gotten about 8,000 people out over the last 24 hours. And if you go back to July, when this effort really started, we’ve gotten about 30,000 people out between our military flights and the charters that we’ve organised.”

Jake Sullivan, the national security adviser, told CNN’s State of the Union “several thousand Americans” were still in Afghanistan but the administration did not know precisely how many.

“We are working hard to make arrangements with each of those people [we have contacted] and make plans to get them out,” he said.

The Civil Reserve Air Fleet was created in the 1950s as a way to help the US military transport people and supplies in times of exceptional stress. All major US airlines are part of it. It has only been activated twice before, during the Gulf war in 1990 and the Iraq war in 2003.

Austin, Sullivan and Blinken all hinted that the Biden administration could extend the 31 August deadline for completing evacuation efforts.

“We’re gonna continue to assess the situation,” Austin said. “And again, work as hard as we can to get as many people out as possible. And as we approach that deadline, we’ll make a recommendation to the president.”

Sullivan was asked how many Afghans were waiting to get out of their country, and if they could be guaranteed that the deadline will be extended if necessary.

“We are working to establish as much order and security outside that airport as possible,” he said, adding that the administration was “not going to rest” until all Afghans in need of help had received it.

Sullivan said the threat from Isis at the airport was “real and acute and persistent and something we are focusing on with every tool in our arsenal”.

“We are working hard with our intelligence community to try to isolate and determine where an attack might come from.”

Blinken would not be drawn on operational plans for protecting Americans and others from terrorist attack. He told CBS: “We’re in direct contact with American citizens and others, and we’re able to guide them best way to get to the airport, what to do when they get there. And that is I think safest, the most effective way to get people there, get them in and get them out.”

Asked if the US was having to “ask permission” of the Taliban in order to get people out, Blinken said: “They are in control of capital. That is the reality.”

Austin was asked why the US military was not sending convoys from the airport to pick up those stuck outside.

He said: “If you have an American passport, and if you have the right credentials, the Taliban has been allowing people to pass safely through.

“[But] there’s no such thing as an absolute. And this kind of environment, as you would imagine, there have been incidents of people having some tough encounters with Taliban.

“As we learn about those incidents we certainly go back and engage the Taliban leadership and press home to them that our expectation is that they allow our people with the appropriate credentials to get through the checkpoints.

“[The] most capable military in the world is going to make sure that airfield remains secure and safe, and we’re going to defend that airfield. We’re going to look at every way – every means possible to get American citizens, third country nationals, Special Immigrant Visa applicants into the airfield.”

Sullivan said the US had “secured the capacity to get large numbers of Americans safe passage through the airport and onto the airfield”. Asked if special operations were being carried out, he declined to comment.

There are signs that the chaos in Kabul, combined with difficulties combating the Covid pandemic at home, has damaged Biden’s public standing. On Sunday, an NBC News poll showed a job-approval rating below 50% for the first time.

Republicans including Donald Trump, who addressed the situation at a rally in Alabama on Saturday night, have seized on events in Kabul as a stick with which to beat Biden.

The Trump administration agreed the US withdrawal with the Taliban in talks from which the Afghan government was excluded. Trump has said he would have withdrawn sooner.