UN Secretary General Antonio Guterres was "shocked" by a letter he received on Friday from Sudan's military ruler General Abdel Fattah al-Burhan regarding the UN's envoy to Sudan, according to his spokesman Stephane Dujarric. The reaction follows reports that Burhan asked that the envoy be removed.

Weeks of fierce fighting in Sudan between two rival groups -- Burhan's Sudanese Armed Forces and the country's Rapid Support Forces led by General Mohamed Hamdan Dagalo -- have left the country in turmoil and scrambled hopes for a peaceful transition to civilian rule.

Earlier on Friday, Burhan had written to Guterres asking that UN special representative to Sudan, Volker Perthes, be removed from his post, Reuters reported citing sources in the Sudanese presidency.

In an address to the UN Security Council earlier this week, Perthes criticized both leaders of Sudan's warring parties and warned of "a growing ethnicization of the conflict."

The conflict shows no signs of slowing down. "Neither side has yet shown the ability to decisively claim a military victory," Perthes said on May 22.

Despite a seven-day ceasefire currently in place -- due to expire this weekend -- fighting has continued between both sides.

Mediators have observed the use of artillery and military aircraft and drones, airstrikes, sustained fighting in the heart of the Khartoum Industrial Area, and clashes in Zalingei, Darfur, according to the US embassy in Khartoum.

Dujarric did not elaborate on the contents of Burhan's letter, but conveyed a brief written statement in support of Perthes: "The secretary-general is proud of the work done by Volker Perthes and reaffirms his full confidence in his special representative."

Speaking to the Security Council on Monday, Perthes also said that the responsibility for the fighting "rests with those who are waging it daily: the leadership of the two sides who share accountability for choosing to settle their unresolved conflict on the battlefield rather than at the table."

The conflict in Sudan has resulted in a heavy toll on civilians, with over 700 people killed, including 190 children, and 6,000 others injured, according to Perthes.

More than a million people have been displaced, seeking shelter in rural areas, other states within Sudan, and crossing Sudanese borders.