Aid workers in South Sudan have suffered an increase in attacks in recent weeks, the UN warned Wednesday, with three killed in May since the formation of a unity government.

Violence continues despite efforts to end a civil war that broke out in December 2013, a conflict that has now seen 55 aid workers killed.

"Violent incidents — including shooting, ambushes, assaults, harassment and robberies — increased during May," the UN Office for the Coordination of Humanitarian Affairs (OCHA) said in a statement.

The three were killed in the Central and Eastern Equatoria regions, areas that escaped the worst of the violence in the war but that have now seen an increase in conflict.

This year, at least 29 aid worker vehicles have been stopped and robbed, and 74 aid agency compounds or offices looted, the UN added.

Civil war erupted in South Sudan in December 2013 but rebel chief Riek Machar returned to the capital in April as part of a peace deal which saw him become vice-president, forging a unity government with President Salva Kiir.

But fighting continues between multiple militia forces who now pay no heed to either Kiir or Machar.

UN peacekeeping chief Herve Ladsous, speaking to reporters on Monday in Juba after a four-day visit to South Sudan, spoke hopefully of a "new page" for the troubled country.

But he also warned of "spoilers" on both sides wanting to "throw a spanner in the works" of the peace process.

More than 158,000 civilians remain in UN-guarded camps across the country, down from a peak of more than 200,000 at the height of the war.