Hillary Rodham Clinton, the US secretary of state, will press South Sudan to resolve disputes with its former rulers in the north, on her first visit to the world's newest country.
Clinton arrived on Friday in South Sudan's capital of Juba for a brief visit to congratulate the new nation on its anniversary and offer US support. It has been reported that she will stress the urgency of ending disputes with Sudan over oil and territory.
Clinton, who is on a seven-nation Africa tour, is the highest-ranking US official to visit South Sudan since it achieved independence last July.
Her visit comes as the two sides had faced a Thursday UN Security Council deadline to reach agreement on the issues or face possible sanctions, but the council deferred action until at least Wednesday.
Clinton will underline US support for the government of South Sudan, which Washington helped guide through years of negotiations with Khartoum that finally led to independence following a decades-long civil war.
But Washington has been dismayed by disputes over border demarcations and oil that at times appeared close to tipping the countries back into war.
Washington wants South Sudan's President Salva Kiir and Sudan's President Omar Hassan al-Bashir to "meet urgently on an on-going basis until all of these issues are resolved", a senior US official said, briefing reporters travelling with Clinton.
Clinton will meet Kiir and will repeat US pledges to help the young nation, one of the world's poorest, find its economic feet, US officials said.
She will fly to Uganda later on Friday for talks with President Yoweri Museveni, a US security ally who nevertheless faces criticism for authoritarian policies at home.
The disputes, particularly over oil revenue, have led to severe economic problems in both Sudan and South Sudan as the oil production has come to standstill.
South Sudan inherited about three-quarters of the region's oil, but shut down its oil industry in January after accusing Sudan of stealing oil that the South must pump through Sudan's pipelines.
That decision has cost both governments dearly in lost revenue but South’s situation is more precarious as it is dependent on oil for about 98 per cent of its state revenues.
In April, the two countries' militaries fought over the disputed, oil-rich region of Heglig.
South Sudan troops took over the town from Sudanese forces, but that offensive maneuver was condemned by world leaders. South Sudan says it then retreated from Heglig, though Sudan says its forces pushed the South out.