Air workers offload food from a UN chopper in an unidentified location in South Sudan.

Violence against aid workers increased over the last one month, the United Nations has said.

The Office for the Coordination of Humanitarian Affairs says three aid workers were killed in May, bringing to 55 the total number of humanitarian staffs killed since the outbreak of the conflict in December 2013.

Violent incidents reported last month include shooting, ambushes, assaults, harassment and robberies.

Seventy-eight humanitarian access incidents were reported, most of which involved violence against personnel or properties.

This was significantly higher than in April, when 48 incidents were reported, and also higher than the monthly average of 63 incidents reported from January to March.

In its latest report, OCHA says road and river travel for aid workers continues to be dangerous: there were 13 incidents of robberies May.

It says aid workers were shot at while attempting to cross the River Nile to deliver assistance between Malakal and Wau Shilluk at the end of the month.

However, OCHA partners have reached thousands of people outside Bentiu with humanitarian aid over the last few months.

Nutrition surveys were conducted in Guit and Rubkona in May and support for malnourished children and pregnant and lactating mothers is ongoing.

The UN Office for Humanitarian Affairs says other areas reached include Guit, Koch, Mayom and Rubkona counties. Vaccination campaigns were done in Nhialdiu, Ding Ding and Thong payams in Rubkona County.

The report also says that about 4,500 internally displaced people have left the UN camp in Bentiu, reducing the number of IDPs to 95,000 at the end of the month.