The travel ban was extended Tuesday to include citizens from Burkina Faso, Mali, Niger, the Palestinian Authority, South Sudan, and Syria, according to the White House.
The European Union has mounted an "air bridge" effort, with at least eight aid flights planned to Sudan. But getting supplies to the areas with the most dire need is an enormous challenge.
South Sudan's government has grounded four aircraft registered under the United Nations mission there, alleging that they were used for illegal surveillance and smuggling.