British Prime Minister Boris Johnson on Wednesday said he was "apprehensive" about the future of Afghanistan as the US announced that its withdrawal from the country was now more than 90 percent complete.

"If you ask me if I feel happy about the current situation in Afghanistan, of course I don't," he told MPs.

"I'm apprehensive, the situation is fraught with risks. We must hope the parties in Kabul can come together to reach an agreement," he added.

The Taliban launched a major assault on Qala-i-Naw in Afghanistan's northwestern Badghis province on Wednesday. The attack by the Islamist group has been seen as an escalation of its assault on the west of the country after the US military began its final drawdown of troops.

Johnson said he hoped "the blood and treasure spent by this country over decades in protecting the people of Afghanistan has not been in vain and the legacy of their efforts is protected."

More than 400 British troops died in action in Afghanistan after the country joined the coalition intervention in 2001.

In 2014 the British mission in the country shifted from a combat operation to one focused on supporting Afghan national forces, with the conflict costing the country around 40 billion ($55 billion, 46.7 billion euros).

"The people of Afghanistan have been the beneficiaries of decades of UK support and investment, we've done our level best to help the stability, security and peace of that country," said Johnson.

"It has been a huge, huge commitment. "

The prime minister said his government will do as much "as we possibly can with our American friends" to encourage peace, and that he would make a statement in parliament on Thursday.

Conservative MP Tobias Ellwood later called for a formal inquiry into the decades-long conflict.

"Two decades after 9/11 we are now abandoning the country to the very insurgent organisation that we went in to defeat in the first place," he wrote on Twitter.