Thousands of Afghans who fled when their country was seized by the Taliban last year face months more in limbo in the United Arab Emirates, a senior US official said Friday.

About 12,000 people have already endured a frustrating wait in the United Arab Emirates capital since the evacuations started in August, when the United States ended its 20-year occupation.

Last month, evacuees held marches in protest at their closed accommodation facility, demanding quicker processing of their cases.

But a senior State Department official, who asked to remain anonymous, said it would take months to clear the two Abu Dhabi centres, which include accommodation built for migrant workers.

"I don't know what's going to happen, but it'd be very nice to have it finished by August," the official said. "But honestly, I can't tell you."

The process has been so slow that the exact number of evacuees in Abu Dhabi remain unclear. A survey of the inhabitants, only cleared to go ahead by UAE officials this year, is still being completed.

"I told them I was really sorry it was taking so long, that I was as frustrated as they were," said the official, after visiting the Abu Dhabi evacuees.

"Several thousand" can hope for resettlement in the US but "very, very, very many of the people came here with no reference to the United States", the official said.

Their path to a host country remains unclear, but the official was confident they would not be "forcibly returned" to Afghanistan, which would be considered illegal.

The evacuees include US citizens and embassy staff and their families, but there are also thought to be members of the CIA-backed paramilitary "Zero units" that human rights groups have accused of abuses such as extrajudicial killings.

"Nobody, nobody comes into the United States without full vetting," the official said, when asked about the alleged Afghan commandos. 

"I don't care who they work for, or what we did or didn't do."

Beyond confusing

Afghans, desperate to avoid a return to the brutal Taliban rule of the 1990s, scrambled on to planes at Kabul airport as the capital fell, some leaving children behind in the melee.

Without holding tickets, they flew to places as far apart as Uganda and Ukraine, in a chaotic evacuation that is causing administrative nightmares months later. Their total numbers are still unknown.

"People got on planes ended up in Albania. People got on planes that ended up in Rwanda. They ended up in Mexico," the official said, calling the situation "beyond confusing".

"I don't know that there was ever a manifest. But there weren't tickets. There weren't seat numbers."

The evacuations to the UAE included "three or four private charters that were approved by the White House" from the northern city of Mazar-i-Sharif, the official said.

At least 100,000 people eligible for US special immigration visas, and their families, are stuck in Afghanistan after the Taliban last week banned further evacuations. Flights had already ground to a halt, with only two since November.

"It's a big problem. I don't know the answer to that, frankly," the official said.