The Angolan authorities have sent some 25,000 people back to the Democratic Republic of Congo, according to a UN-backed radio station.
A local official in south-eastern DR Congo told the AFP news agency that "thousands" of people had crossed the border in recent days.
Some of those expelled say all their goods were stolen.
Angola's ambassador to DR Congo said these were the latest expulsions of illegal migrants from mining areas.
Thousands of illegal diamond miners were deported from Angola in 2004, amid accusations of brutality.
Radio Okapi reports that local officials in the Congolese provinces of Katanga, East and West Kasai, are unable to cope with the influx.
Some 15,000 have arrived in Katanga and about 10,000 in the two Kasai provinces, the radio station said.
Thousands of illegal diamond miners were expelled in 2004
"I went and saw for myself. There is a massive and constant exodus, and humanitarian aid has not yet arrived there," acting West Kasai Governor Hubert Mbingho told Reuters news agency.
David Ntegwe from the UN refugee agency told the BBC's Focus on Africa programme that a UN team had been sent to Katanga to see what the deportees needed, such as food, blankets or water.
Both countries have experienced many years of war, leading thousands of people to cross their common border.