The United Nations on Thursday demanded the release of 12 UN contractors seized by rebels in South Sudan while operating a barge carrying fuel, weapons and equipment for international peacekeepers.

The United Nations on Thursday demanded the release of 12 UN contractors seized by rebels in South Sudan while operating a barge carrying fuel, weapons and equipment for international peacekeepers.

A group of 100 heavily-armed rebels also detained 20 peacekeepers when they captured the barge traveling on the Nile north of Malakal on Monday, but the blue helmets have since been released.

UN spokesman Stephane Dujarric demanded that the "national contractors be released immediately, unharmed and unhurt."

Dujarric also stressed that attacks against peacekeepers may constitute a war crime.

Some 12,500 peacekeepers have been deployed in South Sudan, which has been wracked by conflict since December 2013.

The rebels looted the fuel and detained the contractors in the town of Kaka.

The 20 peacekeepers were flown out of Kaka on Thursday by helicopter to Malakal, said Dujarric.

The United Nations called on rebel leader Riek Machar to "take his responsibilities" and "uphold his commitment to the United Nations that all be returned safely," said Dujarric.