The United Nations says all 20 U.N. peacekeepers detained by rebels in South Sudan have been released.

But U.N. spokesman Stephane Dujarric said Thursday the rebels are still holding 12 U.N. contractors.

He said a U.N. barge carrying fuel was traveling on the Nile river north of Malakal on Oct. 26 when it was blocked in Kaka by over 100 heavily armed men affiliated with the SPLA-in Opposition who detained the peacekeepers and crew. He said the fuel was looted.

Dujarric said the U.N. mission is calling on rebel leader Riek Machar to immediately release the South Sudanese crew "unharmed and unhurt" and return the barge and weapons and equipment belonging to the peacekeepers.

He stressed that attacks on peacekeepers and other U.N. staff can constitute war crimes.