At least four Nigerian peacekeepers in Sudan's western Darfur region have been killed in an ambush, the UN-African Union Mission (UNAMID) says.
The attack, which happened late on Tuesday, also wounded eight peacekeepers in the West Darfur state capital El-Geneina, the peace keeping force said on Wednesday.
"The incident, which involved a Nigerian military patrol, occurred approximately two kilometres [just over a mile] from the mission's regional headquarters," the force said.
"UNAMID personnel, who were heavily fired upon from several directions, returned fire. UNAMID and local authorities are working at the scene of the incident."
Lieutenant-General Patrick Nyamvumba, UNAMID commander, called on the Sudanese government to hunt down those responsible.
"The mission condemns in the strongest terms this criminal attack on our peacekeepers who are here in the service of Darfur's people. I call on the government of Sudan to bring the perpetrators to justice," he said.
Al Jazeera's Harriet Martin, reporting from the capital Khartoum, said: "This is one of the deadliest attacks that UNAMID has experienced, bringing to a total 42 peace keepers killed since the mission was set up at the end of 2007."
"Previous attacks on UNAMID have often boiled down to a desire for either the rebels or Arab militia to get access to weapons but in this case it's not clear if they actually stole anything in this instance," she said.
Genesis of conflict
Ethnic minority rebels rose against the Arab-dominated Khartoum government in 2003. In response, the government unleashed state-backed militia in a conflict that shocked the world and led to allegations of genocide.
The UN estimates at least 300,000 people died but the government puts the toll at 10,000.
UNAMID has been in Darfur for more than four years with a mandate to protect civilians in the vast area the size of France.
As of mid-August, it had lost 38 UNAMID peacekeepers to hostile action since its first deployment.
Although violence is down on its peak, clashes between rebels and government troops, banditry and inter-ethnic fighting continues.
UN figures show that 13 UNAMID vehicles were carjacked in the first half of this year alone.
Key rebel groups refused to sign a deal reached last year between the Khartoum regime and an alliance of smaller rebel splinter factions.
With more than 22,000 international troops and police officers, UNAMID has a budget of about $1.4bn for 2012-13.
The UN Security Council on July 31 expressed "deep concern at increased restrictions and bureaucratic impediments placed by the government of Sudan upon UNAMID movement and operations, particularly to areas of recent conflict".