The decision means South Sudanese could be returned to a nation again on the brink of civil war or unable to seek the U.S. as a haven.

The U.N. had warned on Monday that the country was teetering on the edge of a renewed civil war after fighting in the north between an armed group allied to Machar and government forces.

During the Vietnam War, Da Nang was a major base for both the U.S. and South Vietnamese forces. The first American combat ...