A South Sudanese army general and other officers were also killed in the attack in Nasir, Upper Nile State, where violent clashes with rebels have been occurring for weeks.

The UN team was attempting to extract the armed forces from the area following the escalated political tension.

"The attack on UNMISS personnel is utterly abhorrent and may constitute a war crime under international law," said the Special Representative of the Secretary-General and Head of UNMISS, Nicholas Haysom.

"We also regret the killing of those that we were attempting to extract," Haysom added.

South Sudan, the world's youngest country, ended a five-year civil war in 2018 with a power-sharing agreement between bitter rivals President Salva Kiir and his deputy Riek Machar.

But tensions have been flaring in South Sudan's Upper Nile State, where the government says its army is fighting rebels supported by forces loyal to Machar, the first vice president.

Over the past week, senior officials linked to Machar have been arrested by the government, threatening the country's fragile peace agreement.