The UN on Tuesday condemned the murder of two aid workers in South Sudan and called on authorities to bring their killers to justice following a spate of similar attacks.
The victims were ambushed Monday evening as their convoy returned from delivering food relief in a village some 64 kilometres (40 miles) from Rumbek, in the conflict-prone Lakes State.
They were working for the Italian charity Doctors with Africa CUAMM.
"I call on the government to strengthen law enforcement, investigate these crimes, and to bring the perpetrators swiftly to justice," said Matthew Hollingworth, acting head of the United Nations humanitarian agency OCHA.
"Four aid workers have been killed in the last month alone. I fear that continued attacks on humanitarians and the consequent suspension of activities will have a serious impact on humanitarian operations in South Sudan."
Jacob Akuochpiir Achuoth, health minister for Lakes State, expressed "great sorrow" at the aid workers' deaths and vowed to work closely with investigators to find those responsible.
South Sudan is considered one of the most dangerous countries for aid workers.
The UN, which maintains a peacekeeping mission in the troubled country, says 128 humanitarian workers -- most of them South Sudanese -- have been killed on the job since 2013.
Last month a South Sudanese doctor was murdered in the northern, oil-rich Unity state, barely a month after a nurse was killed in Eastern Equatoria, a south-eastern state.
In January, another aid worker was shot dead near Bentiu, also in Unity state.
The nationality of the latest victims has not been released.
South Sudan achieved independence from Sudan in 2011 but descended into civil war two years later, costing 380,000 lives before a ceasefire was reached in 2018.
The oil-rich country relies on foreign aid and despite ending the war is plagued by armed violence, with clashes between rival ethnic groups claiming more than 1,000 lives in the second half of 2020 alone.