From January to March, 2.8 million people were classed as being in "crisis" or "emergency" food situations, with about 40,000 thought to be suffering an outright famine.

The rising hunger comes despite attempts to end more than two years of war, which started in December 2013 when President Salva Kiir sacked his first vice president Riek Machar, triggering ethnically charged violence.

Some fighting continues, but Kiir was able to name a new cabinet in late April, including former rebels and members of the opposition, after Machar returned to Juba and got back his old job.

"Internal food security analysis shows that South Sudan will face the most severe lean season in 2016 since its independence, driven by insecurity, poor harvests, and displacement in some areas of the country," said a WFP report published on Monday.

"As many as 5.3 million people may face severe food insecurity, with particular areas of concern in the non-conflict affected states of Northern Bahr el Ghazal and Eastern Equatoria."

During the 2015 lean season, which runs from March to September, about 4.6 million people were classed as severely "food insecure", WFP said previously.

The most severe conditions are in Unity State, where a team of food security experts found a risk of "widespread catastrophe" during a visit late last year.

The United Nations says 1.69 million South Sudanese are displaced within the country and another 712,000 have fled into neighbouring countries. The U.N. humanitarian plan for South Sudan has received only 27 percent of the $1.29 billion needed. (Reporting by Tom Miles; Editing by Tom Heneghan)