Attackers killed a Rwandan peacekeeper in an ambush on a UN convoy in conflict-stricken Darfur on Sunday, the United Nations said.
At least 12 UN-African Union peacekeepers have been killed in the troubled Sudanese region in the past five months.
The ambush took place near Kabkayiya in North Darfur, said the UN spokesman Martin Nesirky.
UN Secretary General Ban Ki-moon "was deeply disturbed to learn of another attack today by unidentified armed assailants on a convoy of the African Union-United Nations Hybrid Operation in Darfur (UNAMID)," said Nesirky.
"The secretary general condemns in the strongest terms this attack on UNAMID and expects the government of Sudan to take swift action to bring the perpetrators of this and previous attacks on UNAMID to justice," Nesirky added.
An uprising against the Khartoum government started in the region a decade ago, and the UN says that at least 300,000 people have since died in the conflict.
A new surge in fighting this year has been blamed on rivalry between Arab tribes in the western region. Hundreds have died in clashes between different tribes and militias.
Analysts say the government has lost control of its former Arab tribal allies, whom it once armed against the rebellion, as violent competition for Darfur's resources has intensified.
Ban has repeatedly called on the government to act to halt the violence as the peacekeeping mission and aid groups have become a target in the heightened fighting.
Attackers swooped on a UN water convoy in October and killed three Senegalese peacekeepers. A Zambian military observer was also killed last month. In July, seven Tanzanian soldiers and a Sierra Leone police officer were killed in attack near the Darfur city of Nyala.
Four humanitarian workers have also been killed in the region this year.
The UN says that more than 460,000 people have fled their homes this year because of the conflict.
The UN-African Union mission has more than 25,000 troops and police in Darfur.