The return of Congolese, who had been working in the informal mining sector, came after the Angolan government issued an order to expel them.

"Mass expulsions are contrary to obligations under the African Charter and we ask both sides to respect the human rights of those affected," the UNHCR said in a statement.

There have been unconfirmed reports that a number of these migrants have been brutalised and even killed by Angolan forces before being deported, says the BBC's Louise Dewast in DR Congo capital, Kinshasa.

The reason for this crackdown remains unclear.

For many years, Congolese migrants have been living in Angola, where they dig for diamonds in artisanal mines.

It could be linked to an attempt by the new Angolan government to take control over its mining industry.