Up to 60 heavily armed men on horseback ambushed a patrol of peacekeepers in Darfur, in a new attack on international forces in Sudan's strife-torn west, the United Nations said on Friday.

The raiders, wearing uniforms and armed with AK-47 rifles, rocket-propelled grenades and machine guns, took weapons from Nigerian troops from the joint U.N./African Union Mission in Darfur (UNAMID), an official added.

It was at least the fifth serious confrontation between armed groups and UNAMID troops since they took over from a beleaguered African Union force at the beginning of the year.

The attack happened on Wednesday close to the capital of West Darfur, El Geneina.

U.N. Secretary-General Ban Ki-Moon strongly condemned the attack, according to a U.N. statement released on his behalf on Friday. In it, Ban urged all parties to cease military action and commit themselves to negotiations and a peaceful solution.

UNAMID spokesman Noureddine Mezni said the force had decided not to release details immediately as they were still trying to identify the attackers.

"We have bandits and we have armed groups and we have the (rebel) factions. With our very limited number of troops, it is not an easy job," Mezni told Reuters.

"We are a peacekeeping organisation but there is no peace on the ground to keep. We are appealing for the cooperation of all sides in this conflict. We are here to help."

Mezni said it was unclear how the attackers had taken the UNAMID light weapons. "But the peacekeepers were outnumbered ... No shots were fired and no one was injured."

SERIOUSLY UNDERMANNED

The United Nations has warned that the peacekeeping force remains seriously undermanned -- with only 9,000 out of a promised 26,000-strong force on the ground -- and poorly equipped. The force was sent to keep the peace in a remote region about the size of France.

Law and order has collapsed in Darfur where U.N. officials say five years of conflict have killed up to 300,000 and driven 2.5 million from their homes. Khartoum puts the death count at 10,000 and accuses Western media of exaggerating the conflict.

Nigeria, the country that has contributed the most soldiers to the current force, has borne the brunt of some of the worst violence against peacekeepers in the region.

At least 12 soldiers were killed after armed raiders, thought to belong to a splinter rebel faction, attacked a Nigerian-manned base in the eastern Darfur town of Haskanita in September.

UNAMID is in the process of erecting a memorial stone to around 60 international peacekeepers killed since they first arrived in Darfur on 2004. Ban was "deeply saddened" by the accident, the U.N. statement said.

UNAMID troops held a minute's silence this week in memory of 45 Nigerians killed in a road crash in Nigeria on Wednesday after returning from duties in Darfur.

Troubled peace negotiations between Sudan's government and Darfur rebels were left in ruins this month, when the powerful insurgent Justice and Equality Movement (JEM) launched a shock attack on Khartoum.

Government officials vowed they would never negotiate with JEM after the attack. State media late on Thursday reported the lawyers were finalising cases against a number of people arrested on suspicion of taking part in the May 10 raid on Omdurman, a suburb of Khartoum.

Aid workers running the world's largest humanitarian operation in Darfur have warned that deteriorating security is having a serious impact on their work.

The U.N.'s World Food Programme said it was cutting humanitarian deliveries by half and forced a sharp cut in rations for aid-dependent Darfuris from May after a surge of bandit attacks on its convoys.