Ten UN staff and aid workers abducted in South Sudan almost a week ago have been released.
Three UN staff and seven aid workers, all South Sudanese nationals, went missing last week when their convoy traveling in the Central Equatoria province was hijacked.
The aid workers, employed by a variety of organisations including the South Sudanese Development Organisation, Across, Plan International and Action Africa Help, were flown back to Juba by the International Committee of the Red Cross.
This is the second kidnapping of aid workers in less than a month. Seven aid workers were abducted earlier this month and later released in the same area.
The UN said it is outraged at the deteriorating situation for aid workers in the country.
Last week a humanitarian worker was killed in Leer County, bringing to 100 the number of aid staff killed since the onset of South Sudan's five-year civil war.
"I am deeply concerned by the insecurity faced by aid workers in South Sudan, who are risking their lives to save others," the UN's humanitarian co-ordinator for the country, Alain Noudehou, said.
Opposition forces loyal to former vice-president Riek Machar have claimed responsibility for the most recent abductions, saying the UN is sending humanitarians into rebel-controlled areas without clearance.
"This is undermining the leadership of the (opposition) and it has to stop immediately as it has resulted into misunderstandings and endangering of lives of the workers and our displaced population," opposition spokesman, Lam Paul Gabriel said in a statement.
The opposition's leadership found out about the detained aid workers two days ago and immediately ordered their release, Mr Gabriel said.
But the UN said it never enters into an area to deliver aid until negotiations with all parties are completed.
Human rights groups are calling on all warring factions to stop targeting aid workers and for South Sudan's authorities to investigate the attacks and hold those responsible to account.