Five U.N. peacekeepers and seven civilians working for the U.N. mission in South Sudan were killed in an ambush by unidentified attackers in the restive eastern state of Jonglei on Tuesday, the United Nations said.
"At least nine additional peacekeepers and civilians were injured in the attack and some remain unaccounted for," the U.N. Mission in South Sudan (UNMISS) said in a statement.
India's foreign ministry said the peacekeepers killed were Indian. The nationalities of all the civilians killed were not immediately available, though the United Nations said two of them were South Sudanese and the other five were contractors.
U.N. spokesman Eduardo del Buey said U.N. Secretary-General Ban Ki-moon was appalled by the attack and urged South Sudan "to bring the perpetrators of this crime to justice. He recalls that the killing of peacekeepers is a war crime that falls under the jurisdiction of the International Criminal Court."
The U.N. Security Council and U.S. Ambassador to the United Nations Susan Rice also condemned the attack.
The soldiers were escorting a U.N. convoy near the town of Gumuruk in Jonglei, a remote state hit hard by a cycle of cattle rustling, tribal violence and fighting between government forces and insurgents.
"(The peacekeepers) were in a group of 32 when they were attacked," Syed Akbaruddin, spokesman for India's external affairs ministry, told Reuters. "We came to know five were killed."
Since winning independence from Sudan in July 2011, South Sudan has been struggling to impose its authority across vast swathes of territory teeming with weapons after decades of civil war with Khartoum.
More than 150 people were killed last month in Jonglei, the country's largest state, in a battle between South Sudan's army and insurgents of David Yau Yau, a local rebel leader fighting government forces.
Last month, the African country's army launched an offensive against Yau Yau in Jonglei, where the government hopes to search for oil with the help of France's Total.
Yau Yau mounted a rebellion last year, with support from his Murle ethnic group, after losing local elections in 2010.
A shortwave radio station with links to the Yau Yau rebellion says the group is fighting the government in reaction to abuses committed during a state disarmament programme in Jonglei.
Rights groups accuse South Sudan's army of human rights violations during a disarmament push aimed at ending a cycle of clashes between the Murle and Lou Nuer tribes. The army denies this.
Nearly 900 people died when about 7,000 armed youths of the Lou Nuer tribe attacked Murle villages in the Pibor area at the end of 2011, according to the United Nations.
Hilde Johnson, special representative of the secretary-general in South Sudan, condemned Tuesday's attack.
"This attack will not deter UNMISS and its peacekeepers from working to protect vulnerable communities in South Sudan," she said. "UNMISS is determined to continue its work in supporting authorities ensure peace."