The Mission in South Sudan said on Wednesday it was gravely concerned about the reported execution of four rebels in the north of the country on 7 August.
erupted into civil war shortly after declaring independence from in 2011, which pitted rival communities against each other and plunged the country into violence and lawlessness.
A peace agreement signed three years ago is largely holding but the transitional government has been slow to unify the various factions of the military into a single unit, write a new constitution and pave the way to elections.
The executions were allegedly in response to a 22 July attack by the South Sudan People's Movement/Army (SSPM/A) on the Mayom county commissioner, the United Nations said.
"Anyone who is suspected of a crime must be subject to a proper trial as part of a fair judicial process," the head of the UN mission said in a statement.
Lul Ruai, a spokesperson for the South Sudan People's Defence Forces (SSPDF), said the army had started an investigation into the killings, which it said were captured on a video.
One video shared on social media showed five men in combat fatigues shooting three hooded figures sitting on the ground.
Three men were killed by firing squad while a fourth was tied up and burned alive in a grass-thatched hut, Ruai said in a statement.
"The men were allegedly killed by elements from the SSPDF," he added.
The men were arrested across the border in Sudan on 6 August and handed over to South Sudan authorities, SSPM/A spokesperson Luke Gattiek said in a statement.