About 70 American nationals have been successfully flown from a landing zone at the US embassy in Khartoum to an unspecified location, a second US official said, speaking on the condition of anonymity due to the sensitive nature of the mission.

President Joe Biden ordered the evacuation after receiving a recommendation on Saturday from his national security team with no end in sight to the fighting, the official added.

Embassy evacuations conducted by the US military are relatively rare and usually take place only under extreme conditions.

The US State Department said it has suspended operations at the embassy due to the dire security situation. It is not clear when the embassy might resume functioning.

According to the World Health Organisation, fighting between forces loyal to two top generals has killed more than 400 people since erupting on 15 April.

The violence has included an unprovoked attack on a US diplomatic convoy and numerous incidents in which foreign diplomats and aid workers have been killed, injured or assaulted.

The White House said it has no plans for a government-coordinated evacuation of American citizens trapped in Sudan.

The US embassy said on Saturday that "due to the uncertain security situation in Khartoum and closure of the airport, it is not currently safe to undertake a US government-coordinated evacuation of private US citizens".

An estimated 16,000 US citizens are registered with the embassy as being in Sudan, although that figure is probably inaccurate because there is no requirement for Americans to register or notify the embassy when they leave.

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The conflict between the armed forces, led by General Abdel Fattah al Burhan, and the Rapid Support Forces (RSF), led by General Mohammed Hamdan Dagalo, has derailed Sudan's transition to democratic rule after decades of dictatorship and civil war.

Al Burhan said on Saturday he would facilitate the evacuation of American, British, Chinese and French citizens and diplomats from Sudan after speaking with the leaders of several countries that had requested help.

The RSF in a Twitter posting said it had cooperated with US forces during their evacuation mission.

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Meanwhile, is understood that the UK is planning to evacuate British diplomats as soon as possible.

UK troops and military aircraft have been moved to an overseas base to prepare for what would be a high-risk rescue mission into an active conflict zone - in case the order is given.