Armed raiders killed a policeman and wounded four others in an attack on a refugee camp in Darfur, adding to fears about the safety of displaced people in the war-torn Sudanese region, officials have said.

The attackers fired on a police post at the al-Salam camp in the south of Darfur, the base for thousands of people who have fled their homes during more than four years of revolt.

"This happened yesterday in al-Salam camp," deputy governor of South Darfur state Farrah Mustafa told Reuters from Darfur. "They killed one of our police and injured four."

Mustafa said investigations were continuing into who carried out the attack. He said 26 armed men attacked the post and tried unsuccessfully to steal police vehicles.

International experts estimate 200,000 have died and 2.5 million have been driven from their homes in Darfur after mostly non-Arab rebels took up arms accusing Khartoum of marginalising the remote region.

Khartoum, which puts the death toll at 9,000, mobilised militia to quell the revolt. The International Criminal Court (ICC) has issued arrest warrants for a junior government minister and militia leader accused of conspiring in war crimes.

The world's largest humanitarian operation is active in the region, but aid workers complain a lack of security has hampered efforts and left some 500,000 people out of reach.

Aid officials said it did not appear the camp residents were at risk during the attack, but said they were concerned at the growing presence of arms and insecurity in the region.

"It does not look like that attack was aimed against the civilians in the camp," said Maurizio Giuliano, spokesman for the U.N. Office for the Coordination of Humanitarian Affairs.

"We are concerned by the increasing number of attacks... and the presence of armed men in IDP (internally displaced people) camps," he added.


REBELS SEEK UNITY

The conflict began 4-1/2 years ago in the north of the arid region, but the spread of weapons to other tribes in the south over the past 18 months has increased tensions in these previously safer areas.

Mustafa said this was not the first such attack.

"Five days ago at another station near Kalma Camp (in south Darfur) there was an attack ... they injured four of our police there," he added.

On Sunday, also in South Darfur, a convoy of six commercial vehicles and other cars was attacked about 30 km (19 miles) outside ed-Daieen town in the east of the state, the commissioner of the town told Reuters by telephone.

The attackers were probably rebels or bandits, he said, as they were lightly armed, carrying only two rifles.

The area has witnessed heavy fighting between rebels and militias over the past two weeks.

Since a May 2006 peace deal signed by only one of three rebel groups, the Darfur insurgents have splintered into more than a dozen factions.

But with U.N. and African Union-mediated talks due to start by October, the rebels are trying to unify.

On Sunday a newly formed group, the United Front for Liberation and Development, said its leadership was returning from Eritrea to Darfur to unite their military forces.

"All the leadership council will move to the field in Darfur. The aim is to finish uniting all the armies into one group," UFLD spokesman Abdel Aziz told reporters in the Eritrean capital.