The Afghan capital, Kabul, is not in an “imminent threat environment” despite sudden territorial gains by the Taliban across the country, a U.S. Pentagon spokesperson said on Friday.

Taliban insurgents have seized Afghanistan’s second- and third-biggest cities as resistance from government forces crumbled, stoking fears that an assault on Kabul could be just days away, Reuters reported.

“Kabul is not, right now, in an imminent threat environment. But clearly… if you just look at what the Taliban’s been doing, you can see that they are trying to isolate Kabul now,” John Kirby said.

Also on Friday, a senior U.S. defense official said there was concern that the Taliban – ousted from power by U.S-led forces in 2001 after the 9/11 attacks on the United States – could make a move on Kabul within days.

But Washington was hoping the Afghan security forces would put up more resistance as the insurgents move closer to the capital, Reuters reported.

Kirby said that the Biden administration is concerned by the speed of the Taliban advance.

“We have noted, and we have noted with great concern, the speed with which they have been moving and the lack of resistance that they have faced,” he said.

Afghan First Vice President Amrullah Saleh said after a security meeting chaired by President Ashraf Ghani that he was proud of the armed forces and the government would do all it could to strengthen the resistance to the Taliban.

The fighting has raised fears of a refugee crisis and a rollback of gains in human rights since the Taliban were ousted. Some 400,000 civilians have been forced from their homes since the beginning of the year, 250,000 of them since May, a U.N. official said.

The Pentagon said on Thursday it would send about 3,000 extra troops within 48 hours to help evacuate U.S. Embassy staff.

Pentagon spokesperson Kirby said some of those troops are already in place and the bulk of them would be, “by the end of the weekend.”

The speed of the Taliban offensive as U.S.-led foreign forces prepare to complete their withdrawal by the end of this month has led to recriminations over President Joe Biden’s decision to withdraw U.S. troops.

Biden said this week he did not regret his decision, noting that Washington has spent more than $1 trillion in America’s longest war and lost thousands of troops.