People in Afghanistan should stay away from Kabul airport due to a “high threat of a terrorist attack”, the UK Foreign Office has warned.
In new guidance the FCDO told anyone in the area to “move away to a safe location and await further advice”.
US President Joe Biden has warned of a group linked to Islamic State that is seeking to target the airport.
It comes amid a race to airlift thousands from Kabul before the 31 August when foreign troops will leave.
In the update issued on Wednesday evening, the FCDO warned there was “an ongoing and high threat of terrorist attack”, and that travelling by road was “extremely dangerous”, with people alleged to have been “mistreated” on their way to the airport.
Meanwhile, the US advised Americans in Afghanistan not to travel or gather at the airport “because of security threats outside the gates”.
More than 11,000 people have been evacuated with the help of 1,000 UK troops since 13 August, according to the Ministry of Defence (MoD).
The government says the operation to remove British nationals, as well as Afghans who worked for the UK and other vulnerable individuals, is moving at “significant pace”.
On Tuesday, President Biden rejected calls from Prime Minister Boris Johnson and other allies to delay the withdrawal date for remaining American soldiers beyond the end of the month.
Kabul airport is currently being defended and run by the US, which has 5,800 troops on the ground.
Meanwhile, Defence Secretary Ben Wallace has told MPs that some Afghans wanting to flee to Britain may be better off trying to reach one of the country’s borders and making their way to a third country.
Speaking in an online meeting, Mr Wallace is reported to have signalled there were few places left on evacuation flights and said “not everyone is going to get out”.
But former Tory defence minister Tobias Ellwood, one of the MPs at the meeting, has said any land corridors would have to be agreed with the Taliban if the UK was to avoid the risk of putting people in harm’s way.
Another Conservative MP, Julian Lewis, said he had raised the issue of 12 academics in Afghanistan who are at risk from the Taliban.
Mr Wallace told the MPs that all cases raised with him of people who are in serious danger would be considered by the teams processing urgent requests for evacuation.
Concern over suicide attacks
Analysis by Paul Adams, BBC News
The Foreign Office has previously warned against all travel to Afghanistan, and cited the possibility of terrorist attacks.
But the new advice is very specific. Do not travel to Kabul airport. If you’re in the area, move away to a safe location and wait for further advice.
Officials won’t elaborate on the nature of the threat, but this change comes just 24 hours after President Biden warned of the danger posed by extremists linked to the group calling itself Islamic State.
Commanders dealing with vast crowds around the airport are increasingly concerned about the possibility of suicide attacks.
What the new advice means for the British evacuation operation is not clear.
The MoD said that since 13 August, when the evacuation mission started, a total of 11,474 people had left Afghanistan. That includes almost 7,000 Afghan nationals and their families.
The total also includes British embassy staff, British nationals, those eligible under the UK government’s relocation programme – Afghan Relocation and Assistance Policy (ARAP) – and some evacuees from allied countries.
About 2,000 people eligible for ARAP remain in Afghanistan, but the plan is to evacuate more in the coming days, the BBC was told ahead of the new FCDO advice.
Labour MP Yvette Cooper, who chairs the Home Affairs Select Committee, said the latest government advice was a “further blow to desperate families who have been struggling to get to the airport”.
She said so many people who have worked with the UK government and military were now at grave risk.
The BBC’s chief international correspondent Lyse Doucet said many Afghans would remain unaware of the Foreign Office advice and would continue to make their way to Kabul airport.
Since the Taliban takeover, more than 82,000 people have been airlifted out of Kabul airport, US Secretary of State Antony Blinken has said. About 10,000 are waiting to be evacuated by the US.
Mr Blinken said the Taliban had promised to allow foreigners and Afghans to leave beyond the end of August, adding that the US and its allies had “a responsibility to hold [the Taliban] to that commitment”.
Earlier, Foreign Secretary Dominic Raab said British troops would withdraw from Afghanistan by the end of the month but would not give an exact date when the last UK flight would leave Kabul.
He said the UK wanted to keep using “every hour and day that we’ve got left” to fly British nationals and eligible Afghans out of the country.