South Sudanese troops are preparing to enter the rebel-held town of Bor, President Salva Kiir has said.
Mr Kiir told parliament the army was “ready to move”, adding that the counter-attack had been delayed to allow US citizens to be airlifted out.
Bor, in the restive state of Jonglei, fell to rebels on Wednesday.
A week of ethnic violence has raised fears of civil war. UN humanitarian staff have described bloody scenes, including summary executions.