JUBA Rebels said on Monday the South Sudan government should be held responsible for the killing of six aid workers, the deadliest single assault on humanitarian staff in a three-year-old civil war.
The government said it was too early to say who was behind Saturday's ambush. A U.N. official said on Monday it could amount to a war crime.
The six were ambushed as they were travelling from the capital Juba towards the town of Pibor, the United Nations said, through remote territory largely under government control but fought over by both sides and plagued by militias and other armed groups.
The United Nations did not say which organisation the aid workers belonged to but called on all those in positions of power in South Sudan to stop the violence.
“It will be counterproductive at this stage for anybody to rush for judgment without first allowing the truth to be established," Akol Paul Kordit, the deputy minister of information, told Reuters in Juba.
Rebel fighters loyal to former vice president Riek Machar said the government should be held accountable as the killings took place on its territory.
"We don't have forces in that area. Instead it's the government forces and militias who control that area," said the spokesman for the rebel SPLM-IO forces, Lam Paul Gabriel.
U.N. humanitarian spokesman Jens Laerke said in Geneva the killings could constitute a war crime and must be investigated and prosecuted.
"They are not travelling with escorts, or things like that, they are aid workers, they come unarmed themselves, essentially they have only their humanitarian principles to hold up as a shield," Laerke told Reuters TV.
"All these parties to this conflict must understand that they are providing aid in a neutral, impartial, independent manner, regardless of the areas that are under control of the various parties to the factions.
"They have only one goal, and that is to provide life-saving relief to people who are in desperate need."
Pibor is the main town in Boma state, a vast underdeveloped territory bordering Ethiopia rocked by violence between competing clans this month.
At least 79 aid workers have been killed since President Salva Kiir's government forces clashed with Machar's men in December 2013 - the product of a long-running rivalry between the two men that has split the country along ethnic lines.
U.N. monitors have found Kiir's government mainly to blame for the catastrophe in a country which, in less than six years of independence, has collapsed into a chaotic ethnic war, an epidemic of rape and famine.
(Additional reporting by Marina Depetris; Writing by Clement Uwiringiyimana and Tom Miles; Editing by Andrew Roche)