Rebels in South Sudan have released 13 United Nations contractors after nearly a week of captivity, the UN says.

UN peacekeeping mission in South Sudan (UNMISS) said Monday that the contractors, all South Sudanese nationals, were released on Sunday.

Last week, rebel forces, who have been fighting the government for almost two years, abducted 31 members of the UNMISS. Eighteen Bangladeshi peacekeepers were released soon after they were held captives.

The contractors were on three barges carrying fuel along the Nile River to a UN base in the war-torn northeastern province of Upper Nile.

The UN's top official in South Sudan, Ellen Margrethe Loj, said she was “relieved by the safe release of all UN personnel.”

She had warned on Friday that the captivity "may constitute a war crime.”

According to reports, some 12,500 peacekeepers have been deployed in South Sudan, which has been wracked by the conflict between government forces and rebels since December 2013.

The conflict has killed tens of thousands and displaced more than two million people, with the UN-backed experts warning of the “concrete risk of famine” unless more assistance is provided and access is given to aid agencies to reach the hardest hit areas.

According to the UN, some 3.9 million people -- a third of the country's population -- are in crisis which shows a 80-percent increase in comparison to the same period a year ago.