South Sudan's National Security Service forced several officials assessing the country's new peace deal to strip and face "inhuman" treatment, an international monitoring group said on Wednesday.

Four East African monitors were blindfolded, bound, kicked and robbed of money and a wedding ring on Tuesday, said the Ceasefire and Transitional Security Arrangements Monitoring and Verification Mechanism. The men were stripped to their underwear and the woman was stripped naked, it said. Soldiers threatened to kill their driver and robbed him, it said. All were released five hours later.

The harassment occurred in Luri, just west of the capital, Juba. The monitoring team was investigating alleged recruitment and training of forces, a violation of the peace deal.

Observers call it the worst attack on the international monitoring group since 2016, when a member was killed. The team has also faced beatings, kidnappings and other harassment in the past.

The harassment is a "grave violation" of the peace deal, said the group which added that those involved, including an unidentified brigadier general, should be "held accountable immediately."

South Sudan's government was not immediately available for comment.

The peace deal signed in September was meant to end a five-year civil war that has killed nearly 400 000 people and sent more than 2 million fleeing the country. Its implementation has faced delays, missed deadlines and continued fighting in parts of the country.

The monitoring group is tasked with investigating violations of the peace deal and bringing the warring parties together to build trust before they merge into a single national army. The group is funded by the US, China, Britain, Norway, Denmark, the EU and Japan.

"This incident once again shows that the National Security Service commits abuses as they please and without sanction," said Nyagoah Tut Pur, researcher for the Africa division of Human Rights Watch.

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