resulting in the death of one crew member and several troops. The attack came on Friday as the helicopter was trying to airlift soldiers following heavy clashes in Nasir between South Sudanese forces and the White Army militia, a group which President Salva Kiir’s government has linked to forces loyal to his rival and First Vice President Riek Machar.

“The attack … is utterly abhorrent and may constitute a war crime under international law,” said Nicholas Haysom, the head of UNMISS. “We also regret the killing of those that we were attempting to extract, particularly when assurances of safe passage had been received. UNMISS urges an investigation to determine those responsible and hold them accountable.” The president’s office said Kiir would address the nation on Friday afternoon.

The White Army, whose members are mostly from the Nuer ethnic group, fought alongside Machar’s forces in the 2013-2018 civil war that pitted them against predominantly ethnic Dinka troops loyal to Kiir. Machar’s spokesperson this week said security forces had arrested the petroleum minister, the peacebuilding minister, the deputy head of the army and other senior military officials allied with Machar, raising fears for the country’s fragile peace process. The government has not commented on the detentions and Machar’s party has denied involvement in the fighting in Nasir.