Fighting broke out in December and has driven some one million people from their homes. Photo: BBC

Both the army and rebel forces had recruited the children, she said.

Ms Pillay said South Sudan faced the threat of a famine, but there was an "apparent lack of concern" on the part of its leaders.

She was speaking at the end of a visit to South Sudan, where the conflict has displaced about a million people.

Fighting broke out in December between forces loyal to President Salva Kiir and his sacked deputy, Riek Machar.

Mr Kiir accused Mr Machar of plotting a coup.

'Personal failure'

He denied the charged, but then mobilised a rebel force to fight the government.

The two sides negotiated a truce in January and have resumed talks in Ethiopia, but fighting has continued.

Navi Pillay (C) met government and rebel leaders

Both the government forces and the rebels have been accused of committing war crimes

Ms Pillay said that apart from recruiting child soldiers, both government and rebel forces have carried out "indiscriminate attacks" which killed civilians, including children.

She met both Mr Kiir and Mr Machar during her visit, and is now due to fly to Ethiopia.

"I was appalled by the apparent lack of concern about the risk of famine displayed by both leaders," she said, AFP news agency reports.

"The prospect of widespread hunger and malnutrition being inflicted on hundreds of thousands of their people, because of their personal failure to resolve their differences peacefully, did not appear to concern them very much."

Ms Pillay's visit came amid growing foreign intervention in efforts to end the conflict in South Sudan, the world's newest state.

US Secretary of State John Kerry is due in Ethiopia later on Wednesday, to put pressure on the two sides to negotiate a peace deal.

On Friday, South Sudan's government released four politicians accused of plotting a coup, meeting a key rebel demand.

The UN Security Council has threatened sanctions against those responsible for the continuing violence.

It has condemned the the mass killings of hundreds of civilians in the town of Bentiu, capital of the oil-rich Unity State, earlier this month.

Both sides have been implicated in atrocities and war crimes, and fighting has intensified with the rebels saying they are closing in on northern oil fields and several key towns.

Fighting erupted in the South Sudan capital, Juba, in mid-December. It followed a political power struggle between President Salva Kiir and his ex-deputy Riek Machar. The squabble has taken on an ethnic dimension as politicians' political bases are often ethnic.