Ten aid workers have been released after being abducted in South Sudan almost a week ago, said the U.N. on Monday.

Three U.N. staff and seven aid workers, all South Sudanese nationals, went missing last week when their convoy traveling in Central Equatoria province was hijacked.

The aid workers, employed by a variety of organizations including the South Sudanese Development Organization, 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 U.N. says it's "outraged" at the deteriorating situation for aid workers in the country. Last week a humanitarian was killed in Leer County, bringing to 100 the number of aid workers 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," said the U.N.'s humanitarian coordinator for South Sudan, Alain Noudehou.

Opposition forces loyal to former Vice-President Riek Machar have claimed responsibility for the most recent abductions saying that the U.N. 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, Gabriel told The Associated Press.

But the U.N. says it never enters into an area to deliver aid until negotiations with all parties are completed.

South Sudan's government says the opposition is executing a "mass starvation" strategy for people in the Equatoria region and wants rebel commanders to be held to account by the East African regional bloc leading the upcoming peace talks which will also include Norway, the United Kingdom and the United States, the government's army spokesman, Lul Ruai Koang, told AP.

Right 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.

"Attacks on aid workers have become the norm in South Sudan with dozens killed, injured or have disappeared since the conflict began. This flagrant violation of international law must come to an end," said Seif Magango, Amnesty International's deputy regional director for East Africa, the Horn and the Great Lakes.