AFP/ Juba

A British aid worker in war-torn South Sudan was shot dead in the capital Juba, the government said.

The Briton, who was working for the US aid organisation the Carter Center, was killed by a gunman who followed him into his compound in Juba, according to presidential spokesman Ateny Wek Ateny.

"It has been confirmed that a British staff member in the Carter Center was killed, a crime we condemn in the strongest terms possible," Ateny told AFP.

"He was driving in his car, and when he arrived at his gate he got out of the car, then while walking he was shot," he said, adding that "police have rounded up some people to get some clues."

In London, the Foreign Office confirmed that a British national had been killed.

Aid workers have been targeted multiple times in the 14-month long war, including gunmen shooting down a UN helicopter and peacekeepers killed.

The country is awash with guns, and shots are often heard at night. In October, the armour-plated car of the US ambassador was hit by two bullets.

The Carter Center, set up by former US president Jimmy Carter, is working in South Sudan in several areas, including trying to eradicate guinea worm, a particularly painful water-borne parasite.

- Dependent on aid -

South Sudan is heavily dependent on international organisations for humanitarian aid, with the United Nations estimating that 2.5 million people are in a state of emergency or crisis, just steps short of famine.

International charities have warned of increased harassment, surveillance and threats of expulsion from the government.

In August 2014, gunmen murdered at least six South Sudanese staff members of international aid agencies.

In October, gunmen also abducted two UN workers in separate incidents in the war-ravaged town of Malakal. There has been no news since of either men, both South Sudanese nationals.

Fighting broke out in South Sudan, the world's youngest nation, in December 2013 when President Salva Kiir accused his sacked deputy Riek Machar of attempting a coup.

Over two dozen armed forces -- including government soldiers and allied militia backed by Ugandan soldiers on one side, and a range of rebel factions on the other -- have been battling it out for the last 14-months despite numerous ceasefire agreements.

No overall death toll for the war has been kept by the government, rebels or the United Nations, but the International Crisis Group estimates that at least 50,000 people have been killed.