NAIROBI (Reuters) - South Sudanese security forces detained, stripped, blindfolded and assaulted several foreign monitors of the country’s fragile peace deal earlier this week, according to an internal report by the monitoring body.

Lul Ruai Koang, South Sudan’s military spokesman, said his office was not aware of the alleged incident.

The monitors - army officers from neighboring African countries that endorsed the September accord - had traveled to an area some 20 km (12 miles) west of the capital Juba on Tuesday to investigate an alleged violation of the deal when the incident occurred, the Juba-based group’s report said.

National Security Service personnel detained the monitors and their driver, blindfolding and handcuffing them after forcing them to strip to their underwear, the report read.

The female member of the team was forced to strip naked. They were robbed of money and a wedding ring and detained for five hours before being released, it said.

The report by the Ceasefire and Transitional Security Arrangements Monitoring Mechanism (CTSAMM) called the treatment of its monitors “inhumane” and said the incident was a “grave violation” of the peace deal.

CTSAMM was set up in 2014 to monitor a previous ceasefire in South Sudan by IGAD, a bloc of regional countries. CTSAMM did not immediately respond to a request for comment.

“This is a critical moment for IGAD and the peace process. The response to these early violations will set the tone for the rest of the peace implementation,” said Alan Boswell, South Sudan analyst at Brussels-based International crisis Group.

South Sudanese President Salva Kiir signed a peace agreement with rebel factions in September to end the civil war that erupted in 2013 and killed some 400,000 people, with a third of the population forced from their homes.

Previous peace deals have quickly fallen apart.