The Pentagon is “deeply concerned” about the deteriorating situation in Afghanistan, as the Taliban completed their sweep of Afghanistan’s south on Friday, taking four more provincial capitals in a lightning offensive that brought them closer to the capital, Kabul, just weeks before the US is set to officially end its two-decade war.

The Biden administration has begun rushing 3,000 troops to Kabul’s airport to help with a partial evacuation of the US embassy. The move highlights the stunning speed of the Taliban takeover of much of the country, including their capture on Thursday of Kandahar, the second-largest city and the birthplace of the Taliban movement.

“We are certainly concerned by the speed with which the Taliban has been moving,” the Pentagon press spokesman, John Kirby, said at a briefing on Friday afternoon.

Kirby hesitated for several moments after a reporter asked him if the defense department had been surprised by the speed and ease with which the extremist insurgent force captured great swaths of the country with little resistance, despite years of US assistance to the Afghan government and training for Afghan troops.

“I’m not going to speculate about ‘surprise’,” Kirby said of the most recent developments.

“We’re obviously watching this just like you’re watching this and seeing it happen in real time, and it’s deeply concerning.”

Kirby said Kabul was “not right now in an imminent threat environment”, but added: “If you just look at what the Taliban’s been doing, you can see that they are trying to isolate Kabul. Now what they want to do if they achieve that isolation I think only they can speak to.”

Asked if Kabul was isolated now, Kirby deflected, saying, “I don’t want to get into a special intelligence assessment on the battlefield.”

The Pentagon called on Afghans to unite in the face of the extremist power grab. Kirby said: “No outcome has to be inevitable.”

Joe Biden announced in April that he planned to withdraw all US troops from Afghanistan by 11 September this year, the 20th anniversary of the terrorist attacks on the US in 2001.

The United Nations secretary general António Guterres, on Friday called on the Taliban to immediately halt their offensive in Afghanistan, warning that “Afghanistan is spinning out of control.

“The message from the international community to those on the warpath must be clear: seizing power through military force is a losing proposition. That can only lead to prolonged civil war or to the complete isolation of Afghanistan,” Guterres said.

Molly Montgomery, the deputy assistant secretary of state in the bureau of European and Eurasian affairs, expressed serious concerns for Afghan women and girls as Taliban forces continue to advance towards Kabul.

“Woke up with a heavy heart, thinking about all the Afghan women and girls I worked with during my time in Kabul,” Montgomery said on Friday in a now-deleted tweet.

“They were the beneficiaries of many of the gains we made, and now they stand to lose everything. We empowered them to lead, and now we are powerless to protect them.”

Withdrawing US troops from Afghanistan was a central part of Donald Trump’s “America First” foreign policy, and had the support of a majority of Americans, according to polling.

But many Republicans have nonetheless been highly critical of Biden’s decision to follow through on the withdrawal. The Senate minority leader, Mitch McConnell, compared the Kabul embassy evacuation efforts to “the humiliating fall of Saigon in 1975”

“Afghanistan is careening toward a massive, predictable, and preventable disaster,” McConnell said in a statement released on Thursday.

The House minority leader, Kevin McCarthy, accused the president of failing to “execute a responsible exit”.

“But in a matter of months, the Biden admin has led a botched withdrawal process that has now handed an entire country over to terrorists,” he said on Twitter.