The head of an international group monitoring South Sudan's peace process says he will propose security arrangements to spur the return of rebel leader Riek Machar as vice president.

Festus Mogae told the U.N. Security Council on Friday that it's critical to quickly launch a national unity government to tackle escalating violence, a looming economic crisis and a "humanitarian catastrophe."

Mogae urged the U.N.'s most powerful body to continue playing a leading role in pressing for full implementation of the fragile August 2015 peace agreement between Machar and President Salva Kiir — and to stress that its terms cannot be renegotiated.

Mogae warned the council that "unfortunately, among South Sudan's political leadership the threat to return to war in the event of political dispute is all too frequent."