The US, France and China are also braced to bring out nationals in the face of deadly clashes.

It comes as Sky News understands Britain had put troops and aircraft on standby at an overseas base in case they are needed to airlift embassy staff and UK citizens from the country.

A statement by the Sudanese military citing army chief Abdel Fatteh al Burhan said: "It is expected that the process of evacuation will begin in the coming few hours, as the United States, Britain, France and China will evacuate their diplomats and nationals by air with military transport planes belonging to their armed forces from Khartoum, and it is expected to start immediately."

It follows promises by rival Mohamed Hamdan Dagalo, leader of the paramilitary Rapid Support Forces (RSF), to open airports for evacuations.

Diplomats from Saudi Arabia have already been taken out of the coastal city of Port Sudan and Jordan's diplomats are due to follow suit.

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

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

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

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 the army and RSF wage a power struggle, which has left hundreds of people dead and forced residents to hide in their homes.

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

The spokesperson added: "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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Sudan fighting: Here's what's happening on the ground

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 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."

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.