Sudan is to expel the United Nations's top official in the country after he reported two military defeats for the government and other embarrassing details in the largely invisible war in the western region of Darfur.

Journalists and aid workers have minimal access to the conflict zone to check claims and counter-claims by government and rebel commanders as well as displaced villagers, but Jan Pronk used his authority as UN Secretary General Kofi Annan's special representative to make sensitive statements on his web log, or blog.

This month he reported heavy government casualties, the sacking of several generals and the mobilising of Arab militias to make up for a fall in army morale after frightened troops mutinied.

His remarks were quickly denounced by the Sudanese army, which described Pronk as a security threat, and the Foreign Ministry has told him to leave the country by midday on Wednesday. A spokesperson for Pronk confirmed he will be leaving Khartoum, but added: "He will be going to New York for consultations with the secretary general."

The highly unusual expulsion of a UN official is likely to sour relations between Khartoum and the UN, which were already tense because of Sudan's refusal to accept a Security Council resolution calling for 20 000 troops to move into Darfur to protect civilians. Ironically, Pronk had made it clear he personally agreed with Sudan's position that African Union troops could do the job just as well, provided they had proper funds and equipment.

He was also a critic of the Bush administration for its confrontational line towards Khartoum.

Pronk (66) had a reputation for being outspoken as a minister in two Dutch governments but he took the unusual step of writing a regular blog after his appointment as Annan's special representative in Khartoum two years ago. This seems to have been his main sin. The blog in which he often described the war in Darfur in graphic terms without the usual caution of a diplomat became required reading for everyone watching Sudan's war-torn western region closely.

The Sudanese government has admitted suffering two recent setbacks on the battlefield, at Um Sidir to the north of the main town, El Fasher, and again near the Chadian border two weeks ago. But Pronk's blog gave new details.

"The losses seem to have been very high," he wrote. "Reports speak about hundreds of casualties in each of the two battles with many wounded and many taken as prisoner. The morale in the government army in north Darfur has gone down.

"Some generals have been sacked; soldiers have refused to fight. The government has responded by directing more troops and equipment from elsewhere to the region and by mobilising Arab militia. This is a dangerous development. Security Council resolutions which forbid armed mobilisation are being violated.

"The use of militia with ties with the Janjaweed recalls the events in 2003 and 2004. During that period of the conflict systematic militia attacks, supported or at least allowed by the SAF [Sudanese Armed Forces], led to atrocious crimes."

Foreign Ministry spokesperson Ali al-Sadiq explained the expulsion as resulting from "the latest statements issued by Pronk on his website regarding severe criticism of the Sudanese Armed Forces and the fact that he said the government of Sudan is not implementing the Darfur peace agreement".

Al-Sadiq said rebels would consider Pronk's comments as encouragement to continue their military campaign.

Pronk also annoyed the army by crossing front lines and meeting rebel leaders in Darfur this month, although he made it clear on his blog that he urged them to accept a ceasefire and think about signing the peace deal brokered in May.

He wrote: "In a mass meeting with them -- I counted about 300 military commanders and political officials -- they promised not to attack the Sudanese Armed Forces ... I demanded more: stop considering Amis, the African Union peacekeeping force, as your enemy.

"Guarantee that bandits and rogue commanders no longer harass aid workers and steal their vehicles. Do not reject the Darfur peace agreement, but consider this text as a basis for peace, as a starting point for further talks."