Agencies are focusing assistance on 84,000 displaced people who fled 30 km downstream from Bor, crossed the River Nile to safety and are camped out in Awerial.
BURNED TO ASH
The insecurity and lack of roads make South Sudan a nightmare for humanitarian work.
"We are reliant on planes and helicopters to get us to these locations, which means we are reliant on the UN to assess that it is safe to fly," said Mould.
"Because the fighting is ongoing, it might be safe for 24 hours and then the next day it's unsafe again. What we really need to see is a consistent period of stability to let humanitarian aid into all these areas."
She said a short window of calm would not be enough to deliver the large quantities of aid required to Bor, Bentiu and Malakal.
Last month, the U.N. negotiated "hours of tranquillity" to fly supplies and staff into Malakal and to retrieve medicines from a warehouse in the town.
Malakal was destroyed by intense fighting and most of its 100,000 inhabitants have fled, according to the Danish Refugee Council.
"Most markets have been looted and some burned to ash," the UN humanitarian coordinator in South Sudan, Toby Lanzer, tweeted.
The United Nations said humanitarian access was also being hampered by "attacks on aid workers and assets, interference with humanitarian activities and other obstacles".