Several thousand Rwandan Hutus are being forced home from Burundi against their will, the United Nations fears.
Burundian and Rwandan officials tried to persuade the Hutus to return
They have fled into Burundi since April saying they face persecution by village courts over the 1994 genocide.
The UN refugee agency has protested to Burundi. It has been told 3,700 have already been sent back with the remaining 1,700 going on Monday.
UNHCR staff say they have been kept out of Songore camp, but have seen lorries taking the refugees away.
UN peacekeepers in the area were also told to leave by the local governor.
At a meeting over the weekend, Rwanda and Burundi declared the Rwandans illegal immigrants.
"Nothing justifies the presence of these people in Burundi. Rwanda is in peace and there is no persecution," said Burundian Interior Minister Jean Marie Ngendahayo.
Last week, the UNHCR began building sheds and latrines to accommodate the overcrowded camp in northern Burundi.
It says it would be illegal to return the people before establishing whether they should be granted refugee status.
The Gacaca courts have recently begun trying suspected killers from Rwanda's 1994 Hutu-led genocide that killed some 800,000 Tutsis and moderate Hutus.
Some 12,000 courts are operating based on traditional systems of justice, in which the victims confront their alleged attackers in front of other villagers.
"All the Hutus are guilty to them," a 45-year-old man in the camp had said shortly after arriving in Burundi.