Time is running out, Antonio Guterres told the 15-member Security Council
The UK on Monday promised it was doing "everything we can" to evacuate its citizens from conflict-hit Sudan, as some of those trapped complained they felt "abandoned".
Ferocious battles between the Sudanese army and a paramilitary group — which has seen fighting with tanks in densely populated Khartoum and air strikes launched by fighter jets — have killed more than 400 people and left thousands wounded.
Foreign office minister Andrew Mitchell defended prioritising a night-time military operation to extract embassy staff and their families, saying there had been a "very specific threat to the diplomatic community".
Lawmaker Tobias Ellwood, chairman of a parliamentary defence committee, called for a "clear-cut plan" to get British passport-holders out of Sudan.
"If that plan does not emerge today, then individuals will then lose faith and then start making their own way back," he told the television channel GB News, saying that could lead to "some very difficult situations".
One Briton told the BBC he had been forced to make his own evacuation arrangements, even as other countries got their citizens out of the country.
The man who gave his name as William said he left Khartoum on a bus arranged by his Sudanese employer because "we've had absolutely nothing but nonsense from the Government".
Another, Iman Abugarga, who is in Khartoum, said she felt "absolutely" abandoned by the UK government, which came in for similar criticism after the Taliban took control of Kabul in 2021.
"It is shameful how they have mismanaged this situation," she told the Daily Telegraph newspaper.
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Around 2,000 British passport-holders had contacted the UK authorities to register their presence in Sudan, Mitchell added.
"The situation is absolutely desperate and a ceasefire is what is required," Mitchell said, adding that the "only advice that Britain can give to people is to stay indoors because that is the safe option".
Prime Minister Rishi Sunak on Sunday said UK armed forces had "completed a complex and rapid evacuation of British diplomats and their families from Sudan, amid a significant escalation in violence and threats to embassy staff".
He added that the government was "continuing to pursue every avenue to end the bloodshed in Sudan and ensure the safety of British nationals remaining in the country".
UK forces undertook the military operation alongside "the US, France and other allies", according to Defence Secretary Ben Wallace.
Time is running out, Antonio Guterres told the 15-member Security Council
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