The United Nations has suspended humanitarian operations in southern Sudan after a Save the Children aid agency convoy came under gunfire on Sunday.

Two aid workers, Abhakarel Tayeb and Yacoub Abdelnabi Ahmed, both Sudanese nationals, were killed in the attack.

It's not known who fired on the convoy. According to the UN, the vehicles were clearly marked and recognizable as belonging to Save the Children.

"The fact that the workers themselves seem to have become the target of fighting poses severe difficulties for humanitarian access, with grave consequences for assistance in the future," said UN Special Representative for Sudan, Jan Pronk, in a statement.

Last October, two other Save the Children employees were killed by a landmine in the Darfur region.

The UN has described the conflict in western Darfur between rebel groups and government troops, which has displaced approximately 1.7 million people, as the world's worst humanitarian crisis.