Envoy Jan Pronk said on Wednesday that Sudan Liberation Army (SLA) rebels were responsible for fighting which he said could undermine a separate peace deal to end a longer conflict in Sudan's south.
"The SLA is accountable for a breach not only of the ceasefire but of the recently signed Abuja protocol on security," Pronk said.
Khartoum and both Darfur rebel groups signed a ceasefire in April but both sides have accused the other of repeated violations.
Rebels abandoned Tawilla town in north Darfur on Wednesday after two days of heavy fighting that followed several weeks of skirmishes between militias and rebels.
The United Nations has condemned the fresh fighting, which comes two weeks after both the government and the rebels signed security protocols in the Nigerian capital Abuja.
Asked if Monday's rebel attack on Tawilla rendered the protocols worthless or ineffective, Pronk said: "Worthless - that would have to be seen after the meeting tomorrow [Thursday] of the joint commission. But ineffective? Yes, indeed."
Null and void
One of two main rebel groups operating in Darfur, the SLA said it also considered truce deals signed with Khartoum to be, in effect, over.
"The SLA is accountable for a breach not only of the ceasefire but of the recently signed Abuja protocol on security"
UN envoy Jan Pronk
It warned that it would resume fighting to seek to oust the government.
The statement came from the London-based spokesman of the Sudan Liberation Movement - the political wing of the SLA -a day after he had reaffirmed the SLM's commitment to the accords.
"All the agreements signed in Abuja and Ndjamena have broken down," Mahjub Husain said.
He declared null and void the agreements on a cessation of hostilities signed in Ndjamena last year and a security protocol signed in Abuja earlier this month, saying the government had failed to honour them.
Fighting to continue
The SLM has decided to resume fighting to overthrow the government of Sudanese President Umar al-Bashir, Husain said. "All the war now is starting again. We are ready for everything."
He added that the movement had decided to adopt a "new strategy" and approach in the 21-month conflict with the government and its proxy militias.
The UN has blamed SLA rebels for
breaking the ceasefire
A commission to monitor the ceasefire is due to meet in the Chadian capital on Thursday to review the security situation in Darfur.
Pronk, speaking after an emergency meeting of ambassadors, aid and government officials to discuss the events in Tawilla, said it was possible rebel political leaders did not have full control over the actions of military commanders.
Threat of sanctions
The United Nations has threatened sanctions against Sudan if the government fails to stop the violence in Darfur that it says has led to a crisis in which more than 70,000 people have died since March from malnutrition and disease.
Pronk also urged the government to show restraint and not to let the attacks distract them from a pledge to sign a southern peace deal by the end of the year.
The government and rebels made the pledge last week as the UN Security Council met in Kenya to discuss the Sudan crisis.
"I would be afraid that what's happening at the moment in Darfur might undermine the process of the north-south negotiations," he said.
Negotiations are ongoing between Khartoum and southern rebels to end Africa's longest running war which has killed about 2 million people, mainly through disease and hunger.