Sudan's army has suffered two major military defeats in its campaign against rebels in the Darfur region, the UN envoy to Sudan says.
Mr Pronk said some soldiers were refusing to fight
Jan Pronk wrote on his personal blog there had been hundreds of casualties and prisoners taken, leading to a fall in morale and the sacking of generals.
He also said that pro-government Arab militias were again being mobilised in contravention of UN resolutions.
The Janjaweed militias are accused of widespread atrocities, even genocide.
A former Janjaweed fighter "Ali" now living in London has told the BBC that Sudanese ministers gave express orders for the activities of his unit, which included rape and killing children.
He told the Newsnight programme that Janjaweed fighters would go into Darfur villages after they had been bombed by the air force.
The government has always denied backing the Janjaweed militias and says the problems in Darfur are being exaggerated for political reasons.
More than two million people have fled their homes during the three-year conflict in Darfur.
Mr Pronk said the army's defeats in its latest six-week campaign in Darfur came at Umm Sidir and Karakaya.
Some soldiers were now refusing to fight, he said.
"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."
He also said that Chad, Sudan's neighbour to the west, is also taking an increasing role in the fighting.
Mr Pronk says it seems that Chadian rebels are now fighting alongside the Sudanese armed forces, while Chad's government backs the Darfur rebels.
Envoys from the United States and the UK have visited Sudan this week, but the BBC's Jonah Fisher in Khartoum says the international community is at a loss to stop Darfur's violence.
Our reporter says efforts to persuade Sudan to accept United Nations peacekeepers have failed, with Khartoum seeming determined to pursue a military solution.
A peace deal was signed with one of the rebel factions earlier this year but since then the violence has intensified, with aid workers unable to operate in much of the region.
Mr Pronk says the government has been able to exploit the rebel divisions.
"The rebel movements seem to underestimate how far the DPA [peace deal], if implemented, would restrict the government of Sudan in a possible further abuse of its power," he says.
"It is important to keep what has been achieved, rather than throw away the child with the bathwater."