The South Sudan People’s Defence Forces (SSPDF) has accused First Vice President Riek Machar of obstructing efforts to evacuate soldiers displaced during recent clashes in Nasir County, Upper Nile State.

On Friday, a South Sudanese general and dozens of soldiers were killed when a U.N. helicopter trying to evacuate them from Nasir came under attack, the government said.

The United Nations said the incident, which could deal a blow to an already fragile peace process, was "utterly abhorrent" and a possible war crime.

A crew member was killed and two others were injured when a United Nations helicopter on an evacuation mission in a remote part of South Sudan was attacked.

The aircraft came under fire as it tried to rescue wounded soldiers, the United Nations said. The attack came amid fears that a critical power-sharing deal was at risk of collapse, risking all-out civil war.

President William Ruto has urged South Sudan President Salva Kiir and the first Vice President Riek Machar to engage in dialogue in a bid to foster peace.