The European Union's foreign policy chief has branded developments in Afghanistan "a catastrophe" and said there had been a failure of intelligence to anticipate the Taliban's rapid return to power.

Addressing the European Parliament, Josep Borrell said about 100 EU staff and 400 Afghans working with the EU and their families had been evacuated, but that 300 more Afghans were still trying to leave.

He stressed Europe's "moral duty" to rescue as many Afghans as possible who had worked for the EU in Afghanistan, but said it would not be possible to get them all out.

"Let me speak clearly and bluntly, this is a catastrophe," Mr Borrell said.

"It is a catastrophe for the Afghan people, for the Western values and credibility, and for the developing of international relations."

Western countries have been scrambling to airlift their citizens, Afghan staff and their families out since the Taliban took control of the capital Kabul on Sunday.

Thousands of people have desperately tried to get past Taliban roadblocks and US troops to reach Kabul airport.

The United States has called for safe passage for people trying to reach evacuation flights.

Taliban leaders have in recent days repeatedly vowed not to seek revenge against their opponents, while seeking to project an image of tolerance.

However the US said yesterday that the Taliban were reneging on pledges to allow Afghans who worked with the US and its allies out of the country.

The Taliban urged crowds of Afghans waiting outside the airport to return home, saying they did not want to hurt anyone, a day after the group's fighters fired at protesters, killing three.

In his speech, Mr Borrell addressed concerns that a fresh wave of Afghan migrants might reach Europe, in a replay of the 2015 migrant crisis, when large numbers of people trekked across the continent, many fleeing conflicts in Syria and Iraq.

"Don't call them migrants, they are exiled people, people who are fleeing to save their lives", Mr Borrell said, rejecting comparisons with Syria as Afghanistan is much further away.

Mr Borrell said he was in touch with NATO Secretary-General Jens Stoltenberg, who will host a virtual crisis meeting of the alliance's foreign ministers tomorrow to discuss Afghanistan.

Read more: 12 people killed in incidents around Kabul airport

Meanwhile Britain's Defence Secretary has said the UK is unable to evacuate unaccompanied children from Afghanistan.

Ben Wallace was replying to a question about footage which showed a young child being handed over a wall to Western soldiers at Kabul airport.

The footage, which Reuters was unable to immediately verify, showed a small girl being handed over a wall to British and US soldiers.

Mr Wallace said the child was passed over the wall as her family was being taken out.

"We can't just take a minor on their own," Mr Wallace told Sky News when asked about the footage.

"You will find as you see in the footage I think you're showing now, the child was taken - that will be because the family will be taken as well."

"It will be the challenge trying to make it through that crowd," Mr Wallace said. "We are finding other ways of dealing with that but that is what's happening."

Mr Wallace said British soldiers at the airport were facing a difficult situation given the desperation of some Afghans to leave.

"It is very very difficult for those soldiers as you've seen in your own footage, dealing with some desperate, desperate people, many of whom are just wanting to leave the country," he said.

Mr Wallace said a British presence would stay at Kabul airport as long as US forces continued to run the airport.