Rebels attacked and seized the town of Tawilla in north Darfur state on Monday, and officials said fighting was continuing as police and army struggled to regain control over the town, where about 30,000 refugees are encamped.
The Save the Children UK organisation said about 30 of its workers were forced to flee on foot when a bomb landed about 50m from one of its feeding centres in the town.
Rebel commanders in north Darfur said the town had been bombed, but Foreign Minister Mustafa Usman Ismail denied it.
"There are clear instructions to the army that they should not use any bombing. There is no bombing," he said in Khartoum. But he added helicopter gunships might have been used.
Sudan's security chief has said rebels use villages in Darfur as human shields attracting government bombardment throughout their uprising.
The top UN envoy in Sudan, Jan Pronk, said the rebel attack on Tawilla was a clear violation of the protocols signed earlier this month in the Nigerian capital Abuja between rebels and the government.
He expressed concern that the violence had stopped humanitarian operations around the capital of north Darfur, al-Fasher.
"The parties have committed themselves to refrain from all hostilities and military actions," he said in a statement. "I fully expect them to live up to their obligations."
The UN envoy said the rebels
violated the protocol
After years of skirmishes between nomads and farmers over scarce resources in arid Darfur, rebels took up arms early last year, accusing the government of neglect and of arming militias, known as Janjawid, to loot and burn villages. Khartoum denies the charges.
The United Nations says the violence has caused one of the world's worst humanitarian crises, with about 1.6 million
people forced from their homes in the remote western area.
Saying the rebels had killed more than 30 policemen over the past two days, Ismail lamented the silence of the international community in the face of what he described as the rebels' violations of a shaky ceasefire signed in April.
He said the government had the right to self-defence.
"It is clear that the principle of self-defence is the defence of civilians, and the possessions lost by the civilians and also the defence of governmental organisations who have been attacked," the minister said.
He accused unspecified aid groups of siding with the rebels by not criticising their attacks.
State of emergency
The ministers comments came after a Sudanese government newspaper said that a state of emergency had been declared in the north Darfur state because of attacks by rebels in which many people had died.
The official al-Anbaa daily said local governor Usman Yusuf al-Kabir declared the state of emergency across north Darfur and a curfew in the wake of "a grave military escalation by the rebels".
Ismail said the government had
the right to self-defence
The United Nations has threatened Sudan with possible sanctions if it fails to stop the violence in Darfur.
While there are no reliable figures for the number of killed by the fighting, the World Health Organisation estimates more than 70,000 have died since March from malnutrition and disease.