The US is covering up information about rebels led by a man wanted for war crimes by the International Criminal Court, Human Rights Watch has said.
The global watchdog says Washington is blocking publication of a UN inquiry into rebels led by Bosco Ntaganda in the Democratic Republic of Congo.
The UN has reportedly uncovered detailed information that neighbouring Rwanda, a key US ally, is backing the rebels in the east of the country.
Rwanda has denied the allegations.
The US has denied blocking the report. But it has now been several days since the report by the UN's "Group of Experts" was expected to be published.
The row concerns a rebel group led by Gen Ntaganda, known as "Terminator" and other former officers in the Congolese army. They rebelled from the army and, with their men, now hold territory in parts of DR Congo close to the Rwandan border.
Tens of thousands of people have been made homeless by their recent actions and related military moves by other armed groups.
Most armed groups in eastern DR Congo operate rackets under which they extract precious minerals or "tax" the local population.
The Group of Experts has compiled a report on their activities. This is a long-term investigation into how UN arms sanctions against rebels in DR Congo are respected - or not.
According to sources familiar with the Group of Experts latest report, it contains details of the transfer of weapons from Rwanda to the rebels. There are also reported to be details on how Gen Ntaganda and his allies travel to and from Rwanda in violation of the sanctions.
But Human Rights Watch said the US had used its influence to resist publishing the report's findings.
"The US and other Security Council members should do everything they can," Human Rights Watch said, "to expose violations of UN sanctions, including by Rwanda, and not attempt to cover them up."
Rwanda has strongly rejected allegations that it backs the rebels led by Gen Ntaganda.
The government in Kigali says the main problem in DR Congo is the existence of another armed group which includes people who took part in the genocide in Rwanda in 1994 and then fled to DR Congo. President Paul Kagame of Rwanda repeated this concern at a press conference this week.
The 1994 genocide and its military fallout changed a whole range of strategic relationships in central Africa and is a key to understanding the current row.
Some 800,000 ethnic Tutsis and moderate Hutus were slaughtered by radical Hutu groups. The mass killings of that year were only stopped when Mr Kagame's Tutsi-led army conquered the country.
Some of the policies of the United States and other Western nations towards Rwanda are driven by the guilt they feel for not having stopped the genocide.
Mr Kagame understandably rarely misses a chance to point out that the United Nations and some of its powerful member states failed to help stop the mass killings of his countrymen.
They therefore have no right, he says forcefully at any forum, to preach to him about DR Congo or anything else.
This highly charged background means that whatever happens next to the Experts' report will probably be controversial to one or other of the parties involved.