KAMPALA - U.S. Secretary of State Hillary Clinton will press South Sudan to resolve disputes with its former rulers in the north, on her first visit to the world's newest country on Friday.

On an 11-nation African tour, Clinton will be the most senior U.S. since independence last year, warning that bitter divisions over territory and oil threaten to wreck the economies of two nations.

"We're encouraging both sides, South Sudan and Sudan, to meet and negotiate the differences between them as expeditiously as possible," a senior U.S. official said, briefing reporters travelling with Clinton, who arrived in Uganda on Thursday.

Clinton will underline U.S. 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.

"Both countries are in a downward economic spiral as a result of their sharp political differences," the official said. 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," he said.

Clinton's trip to South Sudan's ramshackle capital of Juba comes one day after the expiry of an August 2 deadline set by the U.N. Security Council for progress in resolving problems between Khartoum and Juba.