"Lieutenant Commander Ashraf Siddiqui, was part of a convoy led by Nepalese peacekeepers providing protection to humanitarian workers travelling from Yei to Lasu when several shots were fired at the group by unknown gunmen," the UN Mission in South Sudan (UNMISS) said in a statement.

The victim, "was hit and died from his injuries shortly afterwards," it added.

The UN said its peacekeepers "immediately returned fire and the assailants retreated into the forest."

"It is a tragedy that he lost his life in such an appalling act of violence while working to help those in need and to protect the lives of others," David Shearer, head of the 13,000-troop force, said.

The attack occurred in the same contested area where 10 South Sudanese aid workers were briefly abducted by rebels in April.

South Sudan has been mired in civil war since December 2013 during which aid workers and peacekeepers have frequently been targeted.

Tens of thousands of people have been killed and millions more uprooted or pushed to the brink of starvation in a conflict characterised by mass rape and the killing of civilians.

UNMISS, for its part, has lost 56 personnel since its deployment began in 2011.

President Salva Kiir and rebel leader Riek Machar are this week meeting in Khartoum, Sudan, in the latest regional effort to end the fighting.