S. Sudan peace deal collapse risks 'all-out war': UN experts

South Sudanese President Salva Kiir (left) and Vice President Riek Machar after peace talk at the State House in Juba, on December 17, 2019. [AFP]

Failure to uphold South Sudan's peace accords could trigger a "catastrophic" return to war and threaten millions of lives, the UN commission documenting rights abuses in the country warned Thursday.

South Sudan's opposition said the arrest overnight of First Vice President Riek Machar, the long-time rival to President Salva Kiir, had invalidated their 2018 peace deal, the cornerstone of the nation's fragile stability.