Only 10,000 troops of a planned 26,000-force have been deployed
UN chief Ban Ki-moon has condemned an ambush which left seven members of the joint UN-African Union peace mission to Sudan's Darfur region dead.
Twenty-two others were injured, seven critically, in one of the deadliest assaults on UN forces in recent years.
The UN says its peacekeepers fought for over two hours to repulse suspected Janjaweed fighters, who were armed with mortars and rocket-propelled grenades.
Only 10,000 of a planned 26,000-strong peacekeeping force have been deployed.
Correspondents say the UN-AU mission, which began work this year, lacks the military hardware, including attack helicopters, needed to operate effectively in a region roughly the size of France.
Khartoum, which wants predominantly African peacekeepers, has been accused of slowing down the deployment of the force by repeatedly raising objections.
UN spokeswoman condemns the killings
About 40 armoured vehicles ambushed the peace force while it was on patrol in North Darfur on Wednesday.
Ten vehicles from the UN-AU Mission in Darfur (Unamid) were destroyed, Sudan's state media reported.
A spokeswoman for UN Secretary General Ban Ki-moon said he "condemns in the strongest possible terms this unacceptable act of extreme violence".
The BBC's Laura Trevelyan at the UN in New York says that UN officials suspect Janjaweed militia loyal to Sudan's government were to blame.
She says diplomats are wondering whether the timing of the attack could be linked to the fact that top Sudanese officials could be indicted for war crimes at the International Criminal Court next week.
The Janjaweed has long been hostile to UN troops in Sudan, fearing they could be used to arrest anyone indicted by the court at the Hague, our correspondent says.
Since the conflict began in Darfur five years ago, the UN estimates that some 300,000 people have died and two million have fled their homes.
The conflict began when rebels took up arms in protest at alleged government discrimination against the region.
Pro-government Arab militias have been accused of widespread atrocities against the black African population.
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