Angola's Catoca diamond mine said a July tailings leak, which turned a river red and killed large numbers of fish downstream in the Democratic Republic of Congo, contained a mixture of sand and clay, but no hard metals.

Congo's government has said the tailings breach killed 12 people and made thousands of others ill, and that it would seek compensation from Catoca for what it called the "toxic" spill.

Catoca, a joint venture between Angolan State diamond company Endiama and Russia's Alrosa, has said its water-sample testing was aimed at refuting Congo's accusations that the spill killed people.

Catoca said in a statement dated October 18 and seen by Reuters on Tuesday that the leak only contained rocks and clay from a tailings basin, which altered the oxygen levels in a tributary of the Congo river.

"Samples taken from wide-ranging sources by independent laboratories confirm that at no point have there been any heavy metals present in river water flowing from the tailings basin," it said.

"By September, the oxygen levels were returning to normal."

Congo's environment minister did not immediately respond to a request for comment. Test results of samples taken by researchers at the University of Kinshasa in Congo's capital are yet to be published.

Catoca, which is the forth largest open pit diamond mine in the world, said it sealed the pipe rupture with three new dykes by the end of July.

It said it will commission independent environmental and structural audits, "to reconfirm that there has been no environmental damage, to minimise the risk of future problems, and to work towards international standards."

Congolese authorities banned communities along the Tshikapa river from fishing or drinking from it, affecting more than 950 000 people and leading the United Nations to issue warnings about the risk of an increase in acute malnutrition among children.