The United Nations peacekeeping operation in South Sudan was severely criticized on Wednesday by Doctors Without Borders, the emergency medical charity, over what it called a shameful indifference to the squalid living conditions of 21,000 displaced people forced to shelter in a flooded portion of a peacekeeping base in the capital, Juba.

The rebuke from Doctors Without Borders was unusual because the charity cooperates with the United Nations in many underserved countries and has been a vital source of aid in South Sudan, which is facing the most severe humanitarian crisis in Africa and perhaps the world, compounded by a political conflict that escalated in December.

About 3.7 million people in South Sudan, a third of the population, are at risk of starvation as the rainy season looms, United Nations officials have said. Hundreds of thousands have been displaced by the conflict.

In a detailed statement, Doctors Without Borders said officials of the United Nations Mission in South Sudan, known by its acronym, Unmiss, had failed to respond to repeated requests by the charity to improve conditions at the Juba base, Tomping, where the displaced live in a low-lying area separated by a barbed-wire fence from empty dry space within the compound.