An official says the South Sudanese military sank a boat carrying 170 of its own soldiers on the Nile river, killing at least 10, after mistaking them for enemy forces.

South Sudan army spokesman Col. Philip Aguer said Sunday that soldiers opened fire after the military vessel failed to respond to repeated calls for it to stop Monday night. Most of the soldiers managed to swim ashore, he said, but 50 are still missing.

Aguer said the South Sudanese government has launched an investigation into what happened, although he described the incident as "an accident."

Relations between South Sudan and Sudan have been marked by tension stemming from disagreements over their common border and how to share oil revenues.

The south became independent of the north last year.