The head of the United Nations mission in Sudan, Jan Pronk, has said the Darfur peace agreement is in danger of collapse and needs re-writing.
Writing his weblog, Mr Pronk called for security guarantees, more disarmament, and more compensation for victims.
He said the pact does not resonate with the people of Darfur, describing it as "severely paralysed".
Only one of Darfur's rebel groups has signed the deal, and many displaced people in the region reject it.
The UN and the African Union have strongly backed the peace agreement over the past two months, describing it as the best hope for the troubled region.
The BBC's Jonah Fischer in Khartoum says Mr Pronk was one of the UN figures who pushed the agreement through, despite objections from all but one of Darfur's rebel factions.
Now he has done a complete U-turn, our correspondent adds.
'Time running out'
In his blog, Mr Pronk said efforts to implement the security elements of the agreement must be redoubled, amid a growing perception that the deal was not worthy of widespread respect.
Commenting on the tight timescales built into the agreement, Mr Pronk said: "It is no wonder that the people in Darfur get the idea that the DPA is just another text without substance, like earlier ceasefire agreements, and is not meant to be kept.
"This only reinforces their rejection of the agreement. It is not yet too late to start implementation, but we seem to be running out of time."
He also called for a broadening of the appeal of the agreement, describing it in its current form as a "necessary but not a sufficient condition for peace".
Minnie Minawi, the rebel leader who signed the deal, is not strong in Darfur, Mr Pronk said, calling for an effort to bring rival leader Abdul Wahid on board.
For saying what has become glaringly obvious, Mr Pronk will have infuriated the Sudanese government and the African Union, our correspondent says.
Sudan has so far refused to allow United Nation's peacekeepers to be deployed into Darfur.
But Mr Pronk was cutting in his judgement of the African Union's record since the agreement was signed in May.
"The African Union is in charge but it clearly lacks the capacity to lead the process of implementation," he said.
The UN is conducting the world's largest humanitarian operation in Darfur, where up to 300,000 people have died in three years of civil war.
A further two million people have been displaced as a result of the conflict and live in miserable conditions in refugee camps.