The UK is beginning a large-scale, complex evacuation of British people from Sudan, PM Rishi Sunak has said.
Families with children, the elderly and people with medical conditions will be prioritised on RAF flights leaving from an airfield near the capital Khartoum.
They are being told to make their own way to the airport, without an escort.
A 72-hour ceasefire, agreed by rival military factions, appears to be holding although there have been reports of new gunfire and shelling.
At least 459 people have been killed since fighting broke out on 15 April.
UK ministers have come under increasing pressure to help its citizens flee the fierce fighting.
Around 4,000 UK citizens are thought to be in Sudan and 2,000 of them have already requested help, Foreign Office minister Andrew Mitchell said on Monday.
Only British passport holders and their immediate family with existing UK entry clearance are eligible, the government has said.
The Foreign Office has told citizens not to make their own way to the airport unless contacted. (BBC)