According to the WFP, severe funding shortfalls mean it can reach just 2.5 million people, only 30 percent of people facing severe hunger.
Another 103,000 South Sudanese have sought refuge in neighbouring countries, pushing the total number of South Sudanese refugees to 2.3 million.
Several South Sudan People’s Defence Forces (SSPDF) members were killed during the United Nations Mission in South Sudan (UNMISS) to extract them from the area.
UNMISS has stepped up efforts to maintain peace in Malakal town, Upper Nile State, following violence that killed several people last week.
Mr Youssef said South Sudan is still recovering from years of conflict.
