Voters in South Sudan overwhelmingly backed a January referendum to become an independent state. The referendum was part of a comprehensive peace deal in 2005 that ended a bloody civil conflict in Sudan.
Princeton Lyman, the United States' new special envoy for Sudan, said there is a long way to go before July 9 when South Sudan becomes the world's newest nation.
"They have a lot of tough issues to negotiate," he said. Issues such as border demarcation and oil revenue remain to be settled. Voters in the oil-rich region of Abyei, meanwhile, didn't vote in the referendum because of conflicts over voting rights.
Clashes between rival groups in Abyei have left more than 100 people dead and displaced at least 20,000 people, the United Nations said.
Lyman added that Washington was "deeply concerned" about ongoing problems in Darfur where nearly 2 million people were displaced by conflict.
Lyman leaves Saturday for meetings in Ethiopia and Sudan to discuss the outstanding issues in the country.