JUBA, South Sudan: South Sudan rebels said Monday the government should be held responsible for the killing of six aid workers, the deadliest single assault on humanitarian staff in a three-year civil war.

The government said it was too early to say who was behind Saturday's ambush, an attack condemned as a "heinous murder" by the United Nations.

The six died as they drove from the capital Juba to the town of Pibor, the United Nations said, through remote territory largely under government control but fought over by both sides in the conflict 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 happened on its territory.

"We don't have forces in that area. Instead its the government forces and militias who control that area," said the spokesman for the rebel SPLM-IO forces, Lam Paul Gabriel.

Pibor is the main town in Boma state, a vast underdeveloped territory bordering Ethiopia rocked by violence between competing clans earlier in March.

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 President Salva Kiir's government is mainly to blame for the catastrophe in his country which – in less than six years of independence – has collapsed into a chaotic ethnic war, with an epidemic of rape and a famine in parts of the country.