No decision has yet been taken for the armed forces to help with any evacuation effort, according to Whitehall sources.

But everyone is "trying to be as ready as possible", one source said on Friday evening.

Sky News has chosen not to disclose the name of the base that is being used as a potential staging post for any rescue mission for security reasons.

British forces could also deploy to other airfields nearer to Sudan.

The United States and France - close allies of the UK - have bases in Djibouti, in the Horn of Africa.

They described the activity within the Ministry of Defence (MoD) as "prudent planning" to prepare for any eventuality.

Prime Minister Rishi Sunak spoke to the president of Djibouti, Ismail Omar Guelleh, on Friday about the situation in Sudan, which was triggered by two warring generals.

"They agreed that the United Kingdom and Djibouti would continue to coordinate efforts to de-escalate the violence and protect civilians, including our citizens," a No 10 spokesperson said.

There are thought to be dozens of UK diplomats and other officials at the embassy in Khartoum as well as hundreds of UK citizens in Sudan. The figure rises into the thousands if dual nationals are also included.

Fierce fighting has gripped Sudan for the past few days as government forces clash with a paramilitary group, leaving dozens of people killed and residents hiding in their homes.

A UK government spokesperson confirmed that the Ministry of Defence is supporting the Foreign Commonwealth and Development Officer "with prudent planning for various contingencies".

The spokesperson said: "We are coordinating across government and with our international partners to provide the best ongoing consular assistance to British nationals and support for our diplomatic staff. We will continue to issue updates as the situation develops."

British nationals are being urged to stay indoors and register their presence in Sudan through a Foreign Office website.

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Mr Sunak chaired a meeting with key ministers on Sudan on Friday.

The Foreign Office is leading the UK's response to the crisis. Foreign Secretary James Cleverly cut short a trip to New Zealand to fly home.

A source said that any movement of troops and military equipment should be seen in the context of careful planning in case required.

The UK could launch its own evacuation mission or join any multinational effort, depending on the level of risk that government ministers were willing to take given that it would involve deploying military forces into a country where active fighting is taking place.

Among the assets available are bigger and smaller transport planes, as well as helicopters.

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UK troops could potentially deploy across the border into the vast country by land or helicopter to facilitate any evacuation.

Summing up the challenge, a defence source said: "There are a lot of people on the ground with a lot of weapons and they do not like each other. At the moment, no one knows who is going to win this particular fight."

The Foreign Office might decide that it is safe enough for its people to remain in place.

"But if it is looking like there could be possible body bags containing diplomatic staff coming back then things become more tense and the risk appetite increases quite dramatically."

Another evacuation option for the UK would be to ask the United States or France, which have larger military footprints in the region, to assist with the rescue of Britons.

The Pentagon has been moving more troops to the African nation of Djibouti to prepare for a possible evacuation of US Embassy staff in Sudan, according to the New York Times.

Underlining the tense and fluid nature of the situation, the British Council - which is separate to the British embassy - said its staff have relocated to their homes in Sudan.

Additional reporting by Jakub Szweda