Military flights will depart from an airfield outside Khartoum and priority will be given to families with children, the elderly and people with medical conditions.

It appears to be race against time as a 72-hour ceasefire began late on Monday - but there are fears over whether it will hold.

Evacuation effort begins for stranded Britons - Sudan latest

"We have started contacting nationals directly and providing routes for departure out of the country," tweeted UK Foreign Secretary James Cleverly.

People will be contacted directly and the government said people should not go to the airfield unless they are called.

"The situation remains volatile and our ability to conduct evacuations could change at short notice," said the Foreign Office.

Sky's Alistair Bunkall - in Cyprus - said a military plane took off from an RAF base on the island this morning to take part in the evacuation.

As well as military flights, the Foreign Office said it is also looking at other potential "points of exit" for UK citizens.

About 4,000 UK passport holders are believed to be trapped in the east African country as rival military factions battle for control.

Hundreds have died since the fighting started on 15 April.