Sudan's army chief Abdel Fattah al-Burhan has accused UN special envoy Volker Perthes of stoking a brutal conflict with paramilitaries, the latest in a series of apparent moves to bolster his war effort.

United Nations chief Antonio Guterres said he was "shocked" by Burhan's letter, which requested "the nomination of a replacement" to Perthes and accused him of committing "fraud and disinformation" in facilitating a political process which broke down into six weeks of devastating urban warfare.

Burhan and his former deputy Mohamed Hamdan Daglo, who commands the paramilitary Rapid Support Forces, were meant to meet for negotiations facilitated by the UN on April 15, the day they turned Khartoum into a war zone.

The meeting aimed to restore a transition to civilian rule disrupted since 2021 when Burhan and Daglo together seized power in a coup before falling out. As their feud worsened, the international community tried to get them to reach a deal on integration of Daglo's RSF into the regular army.

Since late last year Perthes and the UN mission in Sudan, which he heads, have been the target of several protests by thousands of military and Islamist supporters who accused Perthes of "foreign intervention" and demanded his dismissal.

Similar protests have taken place in the eastern city of Port Sudan since the war started.

Perthes had maintained his "optimism" and said the war took him "by surprise".

In the letter, Burhan accused Perthes of bias and of not respecting "national sovereignty."

He said Perthes presented a misleading picture "of consensus" in his reports to the UN, and "without these signs of encouragement, the rebel leader Daglo would not have launched his military operations".

It has never, however, been possible to verify who fired the first shots.

The fighting across Sudan has killed more than 1,800 people, according to the Armed Conflict Location and Event Data Project.

The United Nations says more than a million people have been displaced within Sudan, in addition to 319,000 who have fled to neighbouring countries, raising concerns for regional stability. (AFP)