First the country's army, then its rivals in the paramilitary Rapid Support Forces (RSF) agreed to the three-day extension brokered by the US and Saudi Arabia, it was announced on Thursday evening.

But violence continues in the capital Khartoum and the western Darfur region - with armed groups in the city of Genena battling each other while looting homes and shops, according to eyewitnesses.

The previous truce was due to end at midnight local time (11pm UK time), with the extension set to allow more citizens and foreign nationals to flee the fighting.

The British evacuation mission has rescued at least 897 people, as the White House said it was concerned by ceasefire violations and warned the situation "could worsen at any moment".

Eight British flights had left Sudan as of 4pm on Thursday, with the Foreign Office promising "further flights to come".

Mr Cleverly warned Britons stranded in Sudan it could be "impossible" to evacuate them when the ceasefire expired - as he urged people to head to the airfield north of Khartoum as soon as possible.

More than 2,000 British nationals in Sudan have registered under the evacuation plans. Earlier this week, the UK government said around 4,000 British passport holders were in the country.

British military chiefs say they have the capacity to evacuate at least 500 people per day out of the Wadi Saeedna airfield.

Mr Cleverly tweeted on Thursday evening: "The ceasefire in Sudan has just been extended. The UK calls for its full implementation by the generals. British evacuation flights are ongoing.

"I urge all British nationals wishing to leave to proceed to the airport as quickly as possible to ensure their safety."

Mr Cleverly earlier told Sky's Kay Burley: "We cannot predict exactly what will happen when that ceasefire ends, but what we do know is that it will be much much harder - potentially impossible."

"There are planes, there is capacity - we will lift you out. I'm not able to make those same assurances once the ceasefire has ended."

"So if you're planning to move, move now," he said.

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

However, Mr Cleverly said a "few nationals of other countries" had also been allowed on its planes out of Sudan.

At least 512 people have died and thousands injured since the power struggle between generals heading the army and RSF descended into fighting almost two weeks ago.

Meanwhile, the Foreign Office said the UK ambassador to Sudan, Giles Lever, has been deployed to the Ethiopian capital, Addis Ababa.

He will lead the UK's diplomatic efforts in the region "to bring fighting to an end in Sudan".

The ambassador was not in Khartoum when the fighting broke out with newspaper reports saying he was on holiday at the time.

RAF planes are evacuating people to Cyprus, where they are met by Foreign Office officials and medics, before being flown to London Stansted on commercial jets.

The government is also working on a sea evacuation route from Port Sudan and the HMS Lancaster has been dispatched. Other countries have been using the city to get people out.

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Some have criticised the government for being too slow to start its evacuation plan, with countries such as Germany completing evacuations on Tuesday night.

UK diplomats and their families were evacuated from Sudan in a mission involving elite troops at the weekend.

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The fighting has pushed Sudan's population to near breaking point, with food becoming scarce, electricity cut off across much of the capital and other cities, and many hospitals shut down.

Multiple aid agencies have had to suspend operations and the UN refugee agency said it was gearing up for potentially tens of thousands of people fleeing to neighbouring countries.