Dozens of men on horseback armed with machine guns and rocket-propelled grenades ambushed Nigerian peacekeepers serving with the joint U.N.-African Union force in Darfur, though no casualties were reported, the U.N. said.

About 50 to 60 armed men dressed in military camouflage on Wednesday afternoon ambushed the peacekeepers along the new airport road near El Geneina in West Darfur state, the U.N.-AU mission known as UNAMID said in a statement Thursday. They stole rifles, ammunition, telephones and cash.

UNAMID did not say who was suspected in the attack.

The United Nations peacekeeping chief warned last week of an alarming increase in violence in Darfur that has spread to the Sudanese capital Khartoum and could escalate further. The conflict that began in 2003 has already killed as many as 300,000 people, the U.N. says.

The U.N.-AU force took over peacekeeping duties in the western Sudanese region of Darfur in January from a beleaguered AU force. It only has about 7,500 troops and fewer than 2,000 police on the ground, although a total of 26,000 have been authorized.

The U.N. mission in Sudan has warned that ongoing attacks on the staff and equipment of aid agencies working in embattled Darfur are threatening access to humanitarian aid and worsening already dangerous conditions for Darfur residents.

For example, UNAMID said an armed group hijacked the North Darfur State Water Corporation's drilling rig in March, which could deprive as many as 180,000 of access to clean water this year. UNAMID also said it has sent a fact-finding mission to an area of North Darfur where recent fighting over water resources has led to the reported killing of nine civilians.

The U.N. warning of an alarming increased in violence in the region came after rebels from the Justice and Equality Movement staged an attack in Omdurman, next to Khartoum, earlier this month. The group is based hundreds of miles to the west in Darfur.

The assault took both the government and the international peacekeeping force by surprise and underscored serious shortfalls in the U.N. mission's resources, especially aerial reconnaissance capabilities.

Ethnic Africans in Darfur took up arms against the Arab-dominated government in Khartoum in 2003 to fight discrimination. The U.N. says 2.5 million people have been forced from their homes during the five-year conflict — including 150,000 since the beginning of the year.

The ongoing violence, and the hijacking of dozens of commercial trucks with food from the World Food Program, has forced the U.N. agency to halve rations beginning in May, adding to the concerns about malnutrition in Darfur.