A grenade strike in Sudan's Darfur has killed one aid worker and critically wounded two others as violence worsens in the region , the United Nations said.

Officials in South Darfur state declared a curfew and blamed Thursday's fighting in the region's largest city Nyala on "differences" between members of the security forces.

The previous day in Darfur, assailants fired on an ambulance and wounded three UN peacekeepers during an ambush, the UN's peacekeeping chief said.

The latest incidents add to the worsening security situation in Sudan's far west.

The UN's humanitarian agency OCHA said in a statement that a Sudanese staff member working for an international NGO was killed and three others injured "as a result of a rocket-propelled grenade hitting an NGO office".

"Of the three injured aid workers, two are reportedly in a critical condition and are currently receiving treatment in Nyala hospital."

OCHA said it appeared the NGO office, which it did not name, became caught in the crossfire when fighting erupted in the city at about midday.

"There are also reports that some offices and premises of international organisations in Nyala were looted", it said.

The incident occurred during a resumption of fighting in the centre of the city after the killing overnight of a notorious local bandit who was also a paramilitary officer.

The violence came as United Nations peacekeeping chief Herve Ladsous visited Darfur.

He called the attack by an armed group against the African Union-UN Mission in Darfur (UNAMID) "totally unacceptable".

Ladsous, the UN's Undersecretary General for Peacekeeping, told reporters that "what is even worse is that an ambulance which was with the convoy, an ambulance showing very clearly the red crescent, was shot at. And I could myself touch the holes of the bullets in the ambulance."

Continuous gunfire could be heard over the telephone as AFP spoke with a resident of Nyala on Thursday.

The shooting appeared to be coming from the downtown area where security forces headquarters are located, the resident said, adding that people had to run for cover near the city market.

Fighting began on Wednesday night when a carjacker was killed, leading to retaliation against government forces, sources familiar with the incident said.

After the initial battle, the security forces reinforced their compounds in the city, the sources said.

"There were huge explosions heard, and gunfire" on Wednesday night, a resident said.

The dead bandit was well-known in Nyala by his nickname "Tukron", another resident said.

The brother of the alleged carjacker identified him as Mohammed Abdullah Shurara, a member of the Central Reserve paramilitary police.

Officers from the national security service asked Shurara to report to their office on Wednesday night, "and in front of the office he was shot", said the brother, Ahmed Shurara.

"After that, his men came and attacked the security office," he said.

A UN panel of experts said in February it had collected testimonies about security problems, including "growing crime inside towns such as Nyala".

It said eyewitnesses and victims blamed elements of the Central Reserve and other paramilitaries "for acts of harassment and intimidation" in rural areas or inside camps for the 1.4 million people displaced by Darfur's decade-long conflict.

Rebels have been fighting for 10 years in Darfur.

While the worst of the violence has long passed, instability has been complicated by inter-Arab fighting, kidnappings, carjackings and other crimes, many suspected to be the work of government-linked militia and paramilitary groups.

The UN says an estimated 300,000 more people have been forced to flee their homes this year, primarily because of inter-ethnic clashes.