U.S. Ambassador to the United Nations Susan Rice said much-needed humanitarian aid wasn't getting to Sudan's South Kordofan and Blue Nile states.
"We continue to receive very disturbing reports that tens of thousands of people have become newly displaced due to renewed fighting in the two areas," she said following a U.N. Security Council briefing in early September.
The International Committee of the Red Cross said thousand of refuges have crossed into South Sudan to escape the conflicts.
Melker Mabeck, head of the ICRC delegation in South Sudan, said the situation at some refugee camps is extremely worrying.
"Conditions are dire and survival remains a struggle," he said in a statement. "Owing to the lack of clean water, people are drinking contaminated surface water."
South Sudan gained independence from Sudan last year as part of a comprehensive peace agreement. Ethnic fighting, border conflicts and disputes over oil are threatening the fragile peace.