US Secretary of State Hillary Clinton arrived in South Sudan's capital Juba Friday, the highest-ranking US official to visit the world's newest nation, which split from former civil war foe Sudan last year.

Clinton is due to meet President Salva Kiir and Foreign Minister Nhial Deng Nhial on the three-hour long trip, where she is expected to highlight Washington's concern over the bitter dispute between Juba and Sudan.

Arriving in the steamy heat of the ramshackle Juba -- a rapidly growing city largely made up of simple tin-roof huts strung out alongside the White Nile river -- the top US diplomat headed to the presidency to meet with Kiir.

Juba's government has yet to agree on a raft of issues with the rump state of Sudan, left unresolved after they split in July 2011, including border demarcation and contested areas in oil-rich regions.

The UN Security Council gave the two countries, which earlier this year came close to a return to all-out war, until August 2 to reach a deal or face sanctions. That deadline elapsed Thursday.

"We are encouraging both sides, South Sudan and Sudan, to effectively negotiate the differences between them," said a high-ranking official from the US State Department.

While showing continued support to South Sudan, Clinton "will express our continued concern about the lack of movement in the resolution of the key issues that divide the two countries," the official said.

"These issues are oil and revenue sharing, citizenship, a disputed border," the official added. "Both countries are experiencing economic dislocation."

Clinton's 11-day, seven nation Africa tour is focused on the Obama administration's new Africa strategy of promoting development by stimulating economic growth, while advancing peace and security and strengthening democracy.

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