South Sudan became the world's newest independent country in July under the terms of a 2005 agreement. The deal ended one of the bloodiest conflicts since World War II, though ethnic conflicts, border skirmishes and disputes over oil are threatening the peace agreement.
Pillay, the United Nations said in a statement, is to meet with South Sudan's President Salva Kiir and other top government and civil leaders beginning Tuesday.
She'll discuss the risk to civilians caught up in the hostilities between both countries.
The U.N. Security Council unanimously passed a resolution that calls on Sudan and South Sudan to stop fighting and return to the negotiating table within two weeks.
Fighting flared recently when South Sudan's forces seized the oil town of Heglig near the ill-defined border separating both countries. Kiir had said he wasn't obligated to follow orders from the United Nations when he was called on to pull his forces out of the region in April.
U.S. Ambassador to the United Nations Susan Rice said it was "essential" that both sides return to negotiations under the auspices of an African Union peace panel.
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