The United Nations Children's Fund (UNICEF) says dozens of children have been killed, raped or abducted amid a new bout of fighting between rival militant groups in South Sudan's northern Unity State.

UNICEF Representative Jonathan Veitch said on Tuesday that the children have been targeted over the past two weeks.

“The deliberate targeting of children in these attacks is an outrage. If children are to be protected from further harm, an immediate cessation of hostilities is urgently needed, together with full access for humanitarian workers,” Veitch pointed out.

He further underscored that an urgent and thorough investigation is also required to identify and bring to justice all those responsible for the latest atrocities against children.

The UN official added that some of the assailants were boys as young as 16 or 17 years old, and that some fighters explained that it was better to kill children before they grew up and returned for revenge.

It is an unprecedented use of children in conflict with almost 13,000 children registered to have been recruited by both sides of the South Sudanese conflict, Veitch said.

South Sudan plunged into chaos in December 2013, when fighting erupted between troops loyal to President Salva Kiir and defectors led by his former deputy and current rebel leader Riek Machar around the capital, Juba. The clashes have left tens of thousands of people dead and forced almost two million people from their homes.

Both army soldiers and rebel forces have been accused of widespread atrocities during the violence. The two sides have held several rounds of peace talks, but the negotiations have so far failed to produce a lasting truce deal between the conflicting sides.

South Sudan gained independence in July 2011 after its people overwhelmingly voted in a referendum for a split from Sudan.