The Democratic Republic of Congos state mining firm has signed away the royalties from the countrys most lucrative mining project to a company owned by a close friend of President Joseph Kabila, Global Witness said on Tuesday.

The NGO, which fights corruption and environmental and rights abuses, said Gecamines signed away royalties from Swiss mining giant Glencore for a copper project in the country's southeast to an anonymous Cayman Islands company called Africa Horizons Investment Limited.

It said the January 2015 contract "does not explain what, if anything, Congo's state mining company received in compensation for handing over these rights" from the Kamoto mines.

"The royalties could generate as much as $880 million (820 million euros) -? more than Congo's annual health spending," Global Witness said.

Africa Horizons is owned by Israeli billionaire Dan Gertler, who Global Witness said was involved in murky mining deals in DR Congo and was Kabila's close friend.

"It's troubling that the state miner Gecamines has signed away rights to potentially huge flows of cash that should go towards building Congo's future," Global Witness campaigner Pete Jones said.

"It's even worse that it has handed them to this unknown, anonymous company belonging to an individual with a track record of suspicious deals," he added.

Jones said the contract "provides no reason for Gecamines giving away these royalties".

"It is imperative that Gecamines and Gertler explain what is behind this agreement. If they can't show that this is a good deal for Congo, there should be an investigation into what's really behind the agreement," he said.

Gertler's Fleurette group has denied the accusations and Gecamines director general Deogratias Ngele Masudi told AFP that the firm "does not comment on statements by NGOs even if they are international ones".

DR Congo, a former Belgian territory, has one of Africa's richest mineral reserves but has been wracked by unrest, misrule and corruption since independence.

Kabila took power in 2001, 10 days after the assassination of his father, the then-president, Laurent Kabila.