Most of the remaining British troops assigned to the NATO mission in Afghanistan have left, Prime Minister Boris Johnson announced on Thursday, with the rest to come home shortly.

"All British troops assigned to NATO's mission in Afghanistan are now returning home... I will not disclose the timetable of our departure, but I can tell the House that most of our personnel have already left," he said in a statement to MPs.

Johnson on Wednesday said he was "apprehensive" about the future of Afghanistan as the United States announced that its withdrawal from the country was now more than 90 percent complete.

He stressed to MPs that it would take "the combined effort of many nations, including Afghanistan's neighbours, to help Afghanistan build their future". 

"But the threat that brought us to Afghanistan in the first place has been greatly diminished by the valour and the sacrifice of the armed forces of Britain and many other countries," he said.

"We are safer because of everything they did."

A total of 457 British troops died in action in Afghanistan after the country joined the US-led coalition intervention after the September 11, 2001 attacks in New York and Washington.

The conflict has cost the country around £40 billion ($55 billion, 46.7 billion euros).

In 2014, the British mission in Afghanistan, centred around the restive southern province of Helmand, shifted from a combat operation to one focused on supporting Afghan national forces, with the help of around 750 troops.

In the capital Kabul, fighting raged on Thursday between the Taliban and government forces.

The government flew hundreds of commandos into Qala-i-Naw, in the northwestern province of Badghis, the first provincial capital to face an all-out assault by the Taliban since the US stepped up its troop withdrawal.

Johnson demanded that the Taliban abide by its commitments to a peace deal signed last year.

"I am sure they will be aware there is no military path to victory for the Taliban," he said.

"There must be a peaceful and a negotiated settlement for the political crisis."