The report from the Famine Early Warning Systems Network says starvation is likely to occur in Koch county in the absence of humanitarian aid.
In February, the UN and South Sudan's government officially declared a famine in Leer and Mayendit counties, with a million people said to be at risk.
The new report says famine is likely to spread further during the months of July to September, the lean season for South Sudan's farmers.
The UN and others say the famine is man-made, a result of a three-year civil war that has turned the country into one of the world's largest humanitarian crises. The fighting has blocked aid in some regions, and the government has been accused by aid groups of restricting access. Officials deny it.
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"Humanitarian access is still likely to remain volatile throughout the outlook period in many areas," the new report warns, citing the spread of conflict since fighting erupted anew in the capital, Juba, in July.
On Wednesday, a UN base in Leer county came under attack from the direction of the government-held town, according to the UN. There were no deaths, but aid has been limited as a result of the fighting.
"The people are hungry and deprived as the result of the famine. However, it is clear the attackers have no consideration for their plight, given those who most desperately need help will suffer more because of a likely resulting delay of humanitarian aid," David Shearer, head of the UN mission in South Sudan, said in a statement.