UN backs Sudan envoy Volker Perthes despite letter calling for his removal by Burhan

United Nations chief Antonio Guterres said he was "shocked" by a letter from Sudanese army chief Abdel Fattah al-Burhan, reportedly requesting the replacement of special envoy Volker Perthes amid a brutal war with paramilitaries.

Guterres "is proud of the work done by Volker Perthes and reaffirms his full confidence in his Special Representative", a statement from UN spokesperson Stephane Dujarric said late Friday.

"The Secretary-General is shocked by the letter he received from General Al-Burhan," currently at war with his former deputy Mohamed Hamdan Daglo, who commands the powerful paramilitary Rapid Support Forces.

The rival forces are currently in the fifth day of a one-week ceasefire brokered by the US and Saudi Arabia, during which they have repeatedly accused each other of truce violations.

Neither the army nor the UN have released official copies of Burhan's letter, which reportedly requested the dismissal of Perthes as Guterres' envoy to Sudan.

It is the latest in a series of moves by Burhan, who last week officially sacked Daglo as his deputy in the ruling sovereign council, pooled hardline military supporters into his inner circle and is now seeking to reinforce army ranks.

Sudan's defence ministry on Friday called on "army pensioners, as well as all those capable of bearing arms" to head to their nearest military command unit and "arm themselves in order to protect themselves," their families and their neighbours.

A statement later in the day walked back the call to just army "reservists" and "pensioners".

Perthes and the UN mission in Sudan have been the target of several protests by thousands of military and Islamist supporters who have repeatedly 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 on April 15.

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 300,000 who have fled to neighbouring countries.

Perthes is currently in New York, where he briefed the Security Council on the situation in Sudan earlier this week.

There is no information on when he is due back in Sudan, where authorities have not given out visas to foreign nationals since the war started.