There is no guarantee further evacuation flights will leave Sudan once the ceasefire has ended, the foreign secretary has warned.
The UK has airlifted 536 people from the east African country, and some have arrived in the UK.
But that is a fraction of the thousands of British nationals thought to be in country afflicted by conflict.
“We cannot guarantee how many flights will depart once the ceasefire ends,” James Cleverly said on Wednesday.
“If you plan to leave Sudan please travel to the British evacuation centre as soon as possible.”
The 72-hour ceasefire, which was agreed by Sudan’s army and the paramilitary Rapid Support Forces (RSF), started on Tuesday but ends on Thursday at midnight local time (22:00 GMT).
The Sudanese army said it was willing to send a representative to talks about extending the truce, but there has been no response from the RSF.
The US said it was in direct contact with the two warring generals to see if the ceasefire could be extended.
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Meanwhile in the UK, the first flight from Larnaca airport in Cyprus – where evacuees were flown to initially – landed at Stansted Airport on Wednesday. There were 250 people on board.
Emotions ran high as arrivals looked tired and relieved following a treacherous journey, which started when they left their homes and made their own way to an airstrip north of Khartoum that British troops have taken control of.
Nemar, from west London, who was met by her sister, told reporters: “I am very happy to be here.
“The British government has been marvellous – I feel very proud that I have made it here.”
Shama, one of the first off the airport bus, told reporters and her family: “We’re safe. We’re in no danger – I’m back and no longer scared.”
Asked about the speed of the British response to the violence in Sudan, she said: “It was slow but we’re here.”
Only British passport holders and immediate family members with existing UK entry clearance are being told they are eligible for evacuation.
But Alicia Kearns, the Conservative chairwoman of the Foreign Affairs Select Committee, said elderly people dependent on their children – who themselves have British status – should also be admitted.
She told the BBC: “In the same way we treat children who are dependent on their parents, we should respect that some elderly people are dependent on their children.
“So, I think it is important that we are bringing people out who would otherwise be left destitute and really vulnerable.”
The government is also working to provide other routes out Sudan, with HMS Lancaster travelling towards Port Sudan, the Red Sea dock nearly 500 miles from Khartoum that could be used in a seaborne operation.
But with UK nationals being told to make their own way to evacuations, the trip would be made particularly challenging by fuel shortages and the number of people fleeing.
Senior German political forces said British forces landed in Sudan without permission from the Sudanese army, so angering them they temporarily barred access to the Wadi Saeedna airstrip that European nations were hoping to use.
The UK Ministry of Defence denied it was responsible for any delay.
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