Story highlights 12 aid workers have been killed so far this year in South Sudan

About 5 million people are suffering from famine in the war-torn country

(CNN) Six aid workers were killed in an ambush in South Sudan on Saturday, the UN's humanitarian coordinator for the country said.

The aid workers were traveling from Juba to Pibor, coordinator Eugene Owusu said.

"I am appalled and outraged by the heinous murder yesterday of six courageous humanitarians in South Sudan," Owusu said Sunday.

"These attacks against aid workers and aid assets are utterly reprehensible," said Mr. Owusu. "They not only put the lives of aid workers at risk, they also threaten the lives of thousands of South Sudanese who rely on our assistance for their survival.

He said the ambush comes at a time when humanitarian needs have reached an unprecedented level in the troubled country.

A South Sudanese refugee carries a relief box in Uganda.