Three peacekeepers have been attacked and killed in the North Darfur area of Sudan, the UN and African Union (AU) have said in a statement.
The peacekeepers - from Unamid, the joint UN-AU force - were patrolling in the Zam Zam refugee camp on the outskirts of the town of Fasher.
One assailant also died, the statement said. Six peacekeepers were wounded, three seriously.
Unamid chief Ibrahim Gambari said the attack was a "war crime".
He said the Unamid patrol was "a regular nightly patrol of unarmed police advisers and armed military escorts.
"Their mission was to protect civilians. For that they paid the ultimate sacrifice."
He vowed that justice would be served - but usually attacks on the peacekeepers do not result in arrests or convictions, says the BBC's James Copnall in Khartoum.
The nationalities of the three dead were not immediately given but all were from African nations, said the AFP news agency.
This is the first time such an attack has been seen at the Zam Zam camp, our correspondent says, which highlights the security risks even close to major towns.
Unamid says it has increased the number of patrols because of recent concerns.
A spokesman for one of the main rebel groups accused the government of being behind the attack, but produced no evidence.
Ibrahim al-Hillu, from the Abdul Wahid faction of the Sudan Liberation Army (SLA), also alleged President Omar al-Bashir's trip this week to Qatar was an attempt to obtain weapons to fight in Darfur. Mr Hillu urged the world not to provide arms to Sudan.
There is an arms embargo in place, but activists say it is often flouted, our correspondent says.
Sudanese officials did not respond to requests for comment, but in the past Sudan has said it will take seriously any attacks on Unamid, and has denied breaking the embargo.
The International Criminal Court (ICC) has issued arrest warrants for Mr Bashir over the conflict in Darfur, accusing him of genocide and war crimes. Mr Bashir denies the allegation, saying the ICC is controlled by Western powers hostile to Sudan.
Fighting goes on
Recently the Liberty and Justice Movement, a coalition of minor rebel groups, signed a peace deal with the government in Qatar.
But several major rebel groups did not sign, and fighting continues.
The SLA-Abdul Wahid rebel faction, and others, have agreed to join forces with rebel movements in Blue Nile and Southern Kordofan. But so far there is little sign of any real co-ordination between the rebels, our correspondent says.
The security situation is Darfur is further compromised by the activities of criminals, and armed groups at one time or another allied to the government.
Unamid says 33 Unamid peacekeepers have been killed since the 2007 deployment of the mission, which aimed to stop fighting between rebels and the government in Khartoum.
Some 2.7 million people have fled their homes since the conflict began in Darfur, and the UN says about 300,000 have died - mostly from disease.
Sudan's government says the conflict has killed about 12,000 people and the number of dead has been exaggerated for political reasons.