A major party in South Sudan's coalition government said on Tuesday it had suspended its role in a key element of a 2018 peace deal as relations between its leader Riek Machar and President Salva Kiir deteriorate amid clashes and arrests.

The agreement ended a five-year war between forces loyal to Kiir and his rival Machar, who now serves as First Vice President leading the SPLM-IO party. But the two men have a fractious relationship, which has worsened in recent weeks following clashes in the country's east.

Earlier this month security forces rounded up several SPLM-IO officials, including the petroleum minister and the deputy head of the army, after the White Army ethnic militia forced troops to withdraw from the town of Nasir near the Ethiopian border.

The government has accused the SPLM-IO of links with the White Army, which mostly comprises armed ethnic Nuer youths who fought alongside Machar's forces in the 2013-2018 war against predominantly ethnic Dinka troops loyal to Kiir. The party denies the allegations.

Oyet Nathaniel Pierino, deputy chairman of the SPLM-IO, said on Tuesday the party would not participate in security arrangements tied to the peace process until the detained officials were released.

"The ongoing political witch-hunts continue to threaten the very essence and the existence of the (peace deal)," Pierino said in a statement.

The fighting around Nasir in Upper Nile state has displaced 50 000 people since late February, of which 10 000 have fled to Ethiopia, according to Anita Kiki Gbeho, the United Nations Humanitarian Coordinator in South Sudan.

RAMPANT HATE SPEECH

South Sudan's United Nations peacekeeping chief Nicholas Haysom said he was concerned the country was "on the brink of relapse into civil war".

"With the proliferation of mis/disinformation in the public domain, hate speech is now rampant, raising concerns that the conflict could assume an ethnic dimension," he said in a speech to the African Union.

Analysts say the war in neighbouring Sudan has also spurred the breakdown of the peace process, with South Sudan's oil revenues suspended, escalating regional tensions and arms flooding across the border.

"Already we are seeing the initial stages of spillover fighting in Upper Nile from the Sudan war. It will be difficult to prevent those tensions from spreading to (the capital) Juba," said Alan Boswell from the International Crisis Group.