The Swiss Foundation for Mine Action (FSD) said it did not know who was behind Monday's attack on its workers' convoy near Juba town, but the United Nations said it suspected rebels from the Ugandan Lord's Resistance Army (LRA). "We have suspended operations following the incident until the investigation can provide more information," Henri Leu, president of the Geneva-based group, told a news conference.
"We want to establish whether this was an isolated incident or a sign of deterioration in the security situation."
In a statement, it named one of the murdered employees as Boya Casper, 34, an experienced Iraqi mine clearance expert who took up the Sudan job in September. A Sudanese clearer also killed was not being identified until family had been notified.
They are the first fatalities at the agency, set up in 1997, whose teams have also worked in Bosnia, Iraq and Afghanistan.
FSD deploys 30 expatriates and 250 locals in the Sudan mine clearance programme, which aims to make safe three main access routes from the south into Juba -- more than 600 km (370 miles) of road.
This would open a gateway for its partner, the U.N.'s World Food Programme (WFP), to distribute aid by convoys rather than by costly air freight, and also facilitate refugee returns.
"We vigorously condemn these cowardly assassinations, which create not only great suffering for their families but will have negative consequences for our capacity to rehabilitate the roads of southern Sudan," said WFP spokesman Simon Pluess.
Southern rebels signed a peace agreement with the Khartoum government in January to end more than two decades of civil war. While the vast south has not been heavily mined, heavy rains, hostile terrain and a lack of maps have made locating them next to impossible.
The mine clearers were on a routine trip from the Jebaleen minefield to their campsite when their convoy was attacked, the agency said. Two escorting Sudanese soldiers were also wounded.
The International Criminal Court, investigating war crimes committed during 19 years of conflict in northern Uganda, last month issued its first arrest warrants for five LRA commanders, including elusive leader Joseph Kony.
Kony is thought to be hiding somewhere in southern Sudan.