British nationals who want to leave Sudan have been told "now is the time to move" by the foreign secretary.

James Cleverly said the UK government cannot know what will happen when the ceasefire ends on Thursday at midnight local time (22:00 GMT).

He said people have been able to reach the airfield north of capital Khartoum "in good order" for evacuation, with help from the Sudanese army.

The UK has airlifted 536 people from the east African country.

However, that is a fraction of the thousands of British nationals thought to be in the country.

Mr Cleverly told the BBC he was not able to give an exact number on how many British citizens were currently in Sudan, but urged those who wanted to leave to do so before the ceasefire ends.

"Now is the time to move. We have the aircraft, we have the capacity," he told BBC Breakfast.

"But none of us know what will happen after the ceasefire ends tonight."

The evacuation route from Khartoum to the airfield is being facilitated by the Sudanese army, he added.

"People are successfully getting to the airbase," he said. "It's distressing of course, but they are getting there in good order."

Mr Cleverly said it was not helpful to compare the UK's evacuation response with that of other nations, adding that many British nationals living in Khartoum were widely distributed around the city.

"Where a country has a community which is tightly located - an ex-pat bubble - it is much easier to evacuate en masse," he said.

He said that attempts by some countries to provide support on the route from Khartoum to the airfield had been "prohibitively difficult".

Mr Cleverly has warned there are no guarantees further evacuation flights will leave Sudan once the ceasefire has ended.

"What we do know is it will be much, much harder - potentially impossible," he said.

The 72-hour truce, agreed by Sudan's army and the paramilitary Rapid Support Forces (RSF), began on Tuesday and is due to end on Thursday night.

The US said it was in direct contact with the two warring generals to see if the ceasefire could be extended. 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.

Since fighting erupted on 15 April, the country has been gripped by a power struggle that has seen at least 512 people killed, according to the Sudanese health ministry.

Emotions ran high as among arrivals, following a treacherous journey which started when they left their homes and made their own way to an airstrip north of Khartoum under the control of British troops.

"The British government has been marvellous - I feel very proud that I have made it here," Nemar, from west London, told reporters.

"We're safe. We're in no danger - I'm back and no longer scared," said Shama.

Mr Cleverly defended the pace of the UK's evacuation response of British civilians - which some have judged too slow - telling BBC Radio 4's Today Programme, "we wanted to ensure we didn't put British nationals into increased danger".

"There is a risk to staying put; there is also a risk to moving around in the middle of a conflict," he said.

More than 2,000 British nationals in Sudan have registered with the Foreign Office under evacuation plans.

Only British passport holders and immediate family members with existing UK entry clearance are eligible for evacuation.

But Alicia Kearns, the Conservative chairwoman of the Foreign Affairs Select Committee, said elderly people who are dependent on their children - who themselves have British status - should also be admitted to the UK.

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.

"I think it is important that we are bringing people out who would otherwise be left destitute and really vulnerable."

The foreign secretary said Sudanese nationals without travel documents were being blocked at checkpoints by the Sudanese army.

"Where we have families where a British national has a Sudanese national as a spouse or extended family, it makes the extraction more complicated," said Mr Cleverly.

Downing Street has said that eligibility requirements had been set out "very clearly" and had not changed, but that there was "an element of discretion" for those working on the ground in Sudan.

"We obviously empower people on the ground to make decisions," the prime minister's official spokesman said.

Other evacuation routes out of Sudan are also being worked on. According to tracking websites, HMS Lancaster has arrived at Port Sudan, nearly 500 miles from Khartoum on the Red Sea. But with UK nationals being told to make their own way to departure points, any potential evacuations from there would be complicated by fuel shortages and the volume of people on the roads.

On Thursday, Downing Street said there were no current plans to create a specific resettlement scheme for those fleeing the country.

Meanwhile, the UK has been accused of delaying Germany's efforts to evacuate its citizens from Sudan with its own mission to rescue British embassy staff at the weekend.

The UK Ministry of Defence has denied it was responsible for any delay. The foreign secretary said he would look into Germany's claims.