South Sudan President Salva Kiir is in the United States for an investment conference, his first U.S. visit since his country gained independence in July.
World Bank President Robert Zoellick said during the conference that while gaining independence was a sign of Kiir's commitment to peace, the region has known war and suffering for far too long.
"Although they are now two independent countries, Sudan and South Sudan remain interdependent," he said. "They need to find a path of peace and security, growth and opportunity, together and connected."
South Sudan's independence was secured through a process outlined in a peace deal reached with Washington's help in 2005. Disputes over oil and border conflicts, however, threaten to unravel the peace agreement.
The U.N. refugee agency warned that at least 1,000 people were fleeing violence in Sudan to South Sudan's Blue Nile state.
Sudanese refugees told aid officials it took a month to walk to safety.
Michelle Iseminger, deputy director for the World Food Program's operations in South Sudan, told the United Nations' humanitarian news agency IRIN that supplies were running out.
"We're moving food in as fast as possible," said Iseminger. "It's very difficult because the local places where we get food have been muddy and blocked (and air freight is limited due to several emergencies in South Sudan)."