A United Nations official says that South Sudan's rebels are holding 13 U.N. contractors in Upper Nile state.

U.N. information officer Shantal Persaud told The Associated Press Sunday the U.N. mission is engaging with the rebels to release the men.

The U.N. earlier said 12 local contractors were detained after the rebels released 18 international peacekeepers Thursday. The rebels captured the men last week when they seized a U.N. barge carrying fuel along the Nile River. Rebels control the river's west bank. The U.N. has bases on the government-controlled east side of the river.