The DR Congo army is supposed to be attacking the FDLR rebels
The Democratic Republic of Congo army is collaborating with rebels to mine gold and tin, instead of fighting them, says lobby group Global Witness.
Its researchers found that the two groups operated their own mines and even traded with each other.
The army, with the UN, is supposed to be undertaking a huge operation against the FDLR rebels, accused of taking part in the 1994 Rwanda genocide.
The FDLR presence in DR Congo lies at the heart of years of recent unrest.
Rwanda has twice sent troops into DR Congo, saying it wants to stop FDLR attacks on its territory.
The Congolese defence ministry refused to comment on the allegations but deputy Mines Minister Victor Kasongo told the BBC that action would be taken if the allegations are proven.
He said the army should have no role in the extraction of minerals.
The DR Congo government has promised to wipe out the FDLR and its operations are being backed by UN peacekeepers.
But Global Witness says there are frequent reports of Congolese soldiers selling weapons and uniforms to the mainly Hutu FDLR.
"This complicity extends to the exploitation of minerals," said the group's director, Patrick Alley.
"Our researchers visited areas where the FARDC [DR Congo army] and the FDLR were operating side by side, each controlling their own territories, trading in minerals from 'their' respective mines without interfering with each other's activities. They depend on this mutual support to continue their trade," he said.
The group says the army has been sent to areas where the FDLR operate in North and South Kivu but this has not made much difference.
Mr Alley also said that local people had accused the army of forcing people to work in their mines and extorting money - like the numerous other armed groups which operate in eastern DR Congo.
Global Witness says the FDLR's control of gold and tin mines, especially in South Kivu, gives them the money to continue operating.
It says that until this is stopped, the unrest in the area is likely to continue. There has recently been renewed fighting in the area, between the army and the renegade Tutsi General Laurent Nkunda.
Gen Nkunda has previously refused to disarm, accusing the army of working with the FDLR against Tutsis who live in the region.
Some FDLR leaders are accused of fleeing to DR Congo after taking part in the genocide of Tutsis and moderate Hutus in Rwanda.
Last month, US and European Union diplomats warned that the situation in eastern DR Congo was becoming increasingly tense and that all sides were rearming.
Human rights groups said that tens of thousands of people were fleeing as the situation in the area deteriorated.
The UN has 17,000 peacekeepers in DR Congo, supposed to monitor a 2003 peace deal to end a conflict that drew in at least eight other African countries.
DR Congo is rich in minerals such as gold, tin and coltan, used in mobile phones, but decades of conflict and mismanagement have left the majority of its population living in poverty.