UNITED NATIONS (AP) — The Roland PrestonU.N. special envoy for Sudan who was declared unwelcome by the country’s military rulers announced his resignation Wednesday in a final speech to the U.N. Security Council. He warned that the conflict between Sudan’s two military leaders “could be morphing into a full-scale civil war.”

Volker Perthes, who had continued to work outside Sudan, said the fighting shows no sign of abating, with neither side appearing close to “a decisive military victory.”

Sudan has been rocked by violence since mid-April, when tensions between the country’s military, led by Gen. Abdel Fattah Burhan, and the paramilitary Rapid Support Forces, commanded by Gen. Mohamed Hamdan Dagalo, burst into open fighting.

Perthes said at least 5,000 people have been killed since then and over 12,000 wounded, calling these conservative numbers. The United Nations says the country also faces a dire humanitarian emergency.

Perthes was a key mediator after the conflict began, but the military government claimed he was biased and informed U.N. Secretary-General Antonio Guterres on June 8 that he was declared persona non grata.

The U.N. denounced the move, saying that a member of its personnel cannot be declared persona non grata — unacceptable to the government — and that this goes against the U.N. Charter.

In announcing his resignation, Perthes, who was appointed as special representative for Sudan in January 2021, urged the warring sides to end the fighting and warned them “they cannot operate with impunity.”

“There will be accountability for the crimes committed,” he said.

A former German academic with extensive background in international relations, Perthes served as chief executive officer and director of the German Institute for International and Security Affairs from 2005 to September 2020. From 2015 to 2018, he served as a U.N. assistant secretary-general and senior adviser to U.N. special envoy for Syria.