More than 400 people were killed in South Sudan between January and March this year, with ethnic clashes accounting for much of the violence plaguing the troubled country, the United Nations said Friday.

Clashes erupted on Christmas Eve last year when armed men from Jonglei state, an eastern region beset by gun crime, attacked communities in the nearby Greater Pibor Administrative Area, with the violence later spreading to other parts of the country.

"Intercommunal violence by community-based militias and/or civil-defence groups constituted the primary source of violence affecting civilians", the UN Mission in South Sudan (UNMISS) said in a press release.

"From January to March 2023, the Mission documented 920 incidents of violence against civilians (including 243 children), during which 405 civilians were killed, 235 injured, 266 abducted, and 14 were subjected to conflict-related sexual violence."

Monitors also documented 22 extra-judicial executions between January and May this year, "allegedly committed by South Sudan security apparatus personnel", the statement said.

"I condemn these incidents which occur without any due process," said Nicholas Haysom, the head of UNMISS, adding that the country's justice minister had agreed to investigate the allegations.

One of the poorest countries on the planet despite large oil reserves, South Sudan's leadership has faced fierce criticism for failing its people and stoking violence.

Meanwhile, the crisis unfolding in neighbouring Sudan has forced more than 100,000 South Sudanese refugees to flee back to their homeland, raising fears of fresh ethnic clashes, the UN said, with 13 people killed at a civilian protection camp last week.

Since achieving independence from Sudan in 2011, the world's newest nation has lurched from one crisis to another, including a brutal five-year civil war between forces loyal to President Salva Kiir and his deputy Riek Machar that left nearly 400,000 people dead.

A peace deal was signed in 2018 but sporadic bursts of violence between government and opposition forces continue to occur, while conflict between rival ethnic groups in lawless parts of the country exacts a terrible toll on civilians.

The UN peacekeeping mission in the country is one of the world's most expensive, with an annual budget of $1.2 billion that includes funding for some 17,000 soldiers.