The United Nations on Monday demanded an end to extrajudicial killings in South Sudan after the grisly execution of at least 42 people, including boys, in lawless parts of the country.

In a statement, the UN Mission in South Sudan (UNMISS) revealed that some were executed in front of their families and others left bound to trees in a spate of gruesome lynchings in a country where peaceful governance has remained elusive in the aftermath of civil war.

UN Mission in South Sudan (UNMISS) rights investigators said they have documented the killing of 29 accused criminals in Warrap State since March. The victims, including elderly men and young boys, were taken from prison or police custody and killed without a fair trial.

"Eyewitnesses reported that some men were taken to remote areas, tied to trees, and executed by firing squad. In some instances, their bodies were reportedly left on the trees as an example to the community," UNMISS said in a statement.

The UN said another 13 people were summarily executed since mid-June at the instruction of local officials in Lakes States, a conflict-prone central region.

"People accused of crimes have the right to a fair trial as part of a formal judicial process," said Nicholas Haysom, UN special envoy to South Sudan, in a statement.

"They should not be subjected to the random judgement of government or traditional leaders that they should be taken out and shot in front of their families and communities."

The UN has asked South Sudan's justice ministry to investigate and prosecute those responsible and raised concerns directly with local officials in the two states.

South Sudan has struggled with lawlessness and intercommunal violence since the fighting that left nearly 400,000 dead.

A ceasefire was declared in 2018 but peace remains fragile, with many parts of the country ungoverned and violent.

Haysom said the UN was working with the government and courts to deploy more judges where they were needed. "There is a strong desire among communities for accountability and access to justice. But extrajudicial killings are not a solution to restoring law and order," he said.