Medical charity Medecins Sans Frontieres says South Africa has halted aid for some foreigners displaced during May's xenophobic violence.
According to MSF, some 700 migrants in a camp near Pretoria have been left without security or food.
"This is an attempt to push people out and is completely unacceptable," an MSF spokesman told the BBC.
The authorities want to close all camps hosting the thousands of displaced in the attacks in May by 4 October.
"The authorities at the Acasia camp have stopped providing food and there is no longer any management in the camp," said Jonathan Whittall, programmes director for MSF.
"This is once again an attempt to push people out of the camp and is completely unacceptable."
The United Nations says that around 80,000 were driven from their homes during May's attacks in which 62 people were killed.
But the Consortium for Refugees and Migrants in South Africa puts the number of displaced at 100,000.
Court ruling
Most of the people in the Acasia camp are from Somalia, Ethiopia and Burundi and MSF says many have refugee status.
DISPLACED BY VIOLENCE
Earlier this week, South Africa closed three camps around Johannesburg, holding 1,200 foreigners driven from their homes in May.
This is despite a ruling from the Constitutional Court to keep them open.
It ruled that a full hearing should decide if it was legal to shut them.
The provincial government in Gauteng argues it is now safe for the foreigners to return to their homes.
The Gauteng provincial government's acting chief director for community development, Russell McGregor, said there was no chance of reopening the camps.
The violence - which caused the worst bloodshed in South Africa since the end of apartheid in 1994 - began in a township north of Johannesburg before spreading to other parts of the country.
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