US national security adviser Jake Sullivan says the Afghan military is to blame for the Taliban's swift takeover of Afghanistan.

Mr Sullivan said on Monday US President Joe Biden did not want the US to enter a "third decade of conflict" in Afghanistan and believed it was time for the Afghan army to defend the country two decades after billions of dollars of investment and training by the US.

"At the end of the day, despite the fact that we spent 20 years and tens of billions of dollars to give the best equipment, the best training and the best capacity to the Afghan security forces, we could not give them the will and they ultimately decided that they would not fight for Kabul and they would not fight for the country," he said.

He added the "worst-case scenario" for the US would be to send thousands of troops to fight in a civil war when the Afghan army "wasn't prepared to fight itself".

Mr Sullivan said Mr Biden faced "bad choices" on the subject. 

The President ultimately opted to bring US troops home and leave the Afghans to fight for themselves.

Mr Sullivan said "it's heartbreaking" to see what was happening in Kabul but Mr Biden "stands by" his decision.

US focused on securing Kabul airport

The United States will focus on securing the Kabul airport and additional US forces will flow into the airport on Monday and Tuesday, US deputy national security adviser Jon Finer said on Monday.

The United States had temporarily halted all evacuation flights from Kabul to clear people who had converged on the airfield, a US defence official said, but they did not say how long the pause would last.

The defence official said the United States was intent on getting tens of thousands of at-risk Afghans who worked for the US government out of Afghanistan and was looking at temporarily housing them at Fort McCoy in Wisconsin and Fort Bliss in Texas.

Mr Biden and other top US officials have been stunned by the pace of the Taliban's nearly complete takeover of Afghanistan, as the planned withdrawal of American forces urgently became a mission to ensure a safe evacuation.

Getting out of Afghanistan

The speed of the Afghan government's collapse has seen Mr Biden become the target of withering criticism from Republicans who have said that he has failed.

Mr Biden campaigned as a seasoned expert in international relations and has spent months downplaying the prospect of an ascendant Taliban while arguing that Americans of all political persuasions have tired of a 20-year war, a conflict that has demonstrated the limits of money and military might to force a Western-style democracy on a society not ready or willing to embrace it.

By Sunday, though, leading figures in the administration acknowledged they were caught off guard with the utter speed of the collapse of Afghan security forces. 

Mr Biden is the fourth US president to confront challenges in Afghanistan and he has insisted he will not hand America's longest war to his successor. 

But the President will likely have to explain how security in Afghanistan unravelled so quickly, especially since he and others in the administration insisted it would not happen.

As recently as last week, Mr Biden publicly expressed hope Afghan forces could defend their country. 

But privately, administration officials warned the military was crumbling, prompting Mr Biden on Thursday to order thousands of American troops into the region to speed up evacuation plans.

AP