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 plane took off from an RAF base on the island just before 7am UK time to take part in the evacuation.

The Foreign Office said it is also looking at other potential "points of exit" - possibly by sea, according to Bunkall.

About 4,000 UK passport holders are thought 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 and the evacuation comes after days of pressure for the government to launch a plan to get Britons out.