Gunmen in South Sudan have shot dead two aid workers, colleagues said Wednesday, taking the number killed in over two years of civil war to more than 50.

Gunmen in South Sudan have shot dead two aid workers, colleagues said Wednesday, taking the number killed in over two years of civil war to more than 50.

The pair, South Sudanese nationals who worked for the Danish Demining Group (DDG), making safe landmines and unexploded bombs, were killed in the southern Yei region as they went to work.

"The vehicle was ambushed and two of the team members were shot," DDG said in a statement, calling the killings "tragic."

At least 51 aid workers have been killed since war broke out in December 2013 in South Sudan, according to the United Nations, along with tens of thousands of other people.

Civil war broke out in December 2013 when President Salva Kiir accused his former deputy Riek Machar of planning a coup, setting off a cycle of retaliatory killings that have split the poverty-stricken, landlocked country along ethnic lines.

The conflict now involves multiple militia forces driven by local agendas or revenge.

However, hopes are high that the expected arrival of rebel chief Machar to the capital Juba next week as part of a much delayed peace deal will help bring and to the fighting.