US President Joe Biden said late Sunday Washington has an unwavering commitment to getting American citizens and at-risk Afghans out of Afghanistan.

Biden said the security situation in Afghanistan was changing rapidly and his administration was concerned about the threat from Islamic State (Daesh) in Afghanistan.

"Let me be clear, the evacuation of thousands from Kabul is going to be hard and painful" and would have been "no matter when it began," Biden said in a briefing at the White House.

"We have a long way to go and a lot could still go wrong."

Biden said he had directed the State Department to contact Americans stranded in Afghanistan by phone, email and other means, and the United States had a plan to move them to the airport.

"We're executing a plan to move groups of these Americans to safety and effectively move them to the airport compound. For security reasons, I'm not going to go into detail ... but I will say again today what I've said before: Any American who wants to get home will get home."

Afghan allies of the West and vulnerable Afghans such as women activists and journalists would be helped too, he said.

Asked by a reporter whether the United States would extend an Aug. 31 deadline for evacuations, Biden replied: "Our hope is we will not have to extend but there are going to be discussions I suspect on how far along we are in the process."

Meanwhile, President Vladimir Putin on Sunday rejected the idea of sending evacuees to Russia-allied countries to the north of Afghanistan, saying he did not want "militants showing up here under cover of refugees", Russian news agencies reported.

Putin criticized the idea of some Western countries relocating refugees from Afghanistan to neighboring Central Asian countries while their visas to the United States and Europe are being processed.

"Does that mean that they can be sent without visas to those countries, to our neighbours, while they themselves (the West) don't want to take them without visas?" TASS news agency quoted Putin as telling leaders of the ruling United Russia party.

Taliban spokesman Suhail Shaheen meanwhile said from Doha that all foreign troops should leave Afghanistan by the end of August, Deutsche Welle reported.

Shaheen has said that the withdrawal of all foreign troops from Afghanistan by the end of August is the "red line" of the Taliban, and the group sees the extension as a "continuation of the occupation of the country."

Shaheen added that there was no reason to extend the deadline. Shaheen stressed that the extension of the US and other countries' military presence in Afghanistan is fueling distrust.

According to Deutsche Welle while setting an ultimatum for the withdrawal of foreign troops, he said that extending the deadline for the withdrawal of troops from Afghanistan would face a "reaction" from the Taliban.

Shaheen also said that citizens who had gathered at Kabul airport wanted to leave the country to escape poverty, citing fear of the Taliban as an excuse.