Rwanda and the Democratic Republic of Congo are engaged in a proxy war with each providing assistance to the other's enemies, a report by a United Nations Security Council panel has said.
The report, which was authored by five outside experts, says that Rwanda has been assisting Laurent Nkunda, a Tutsi rebel leader, in his fight against DR Congo government forces in the east of the country.
Meanwhile, DR Congo is providing help to ethnic Hutu rebels and other forces against Rwanda, the report says.
The panel also said it had evidence that armed groups from both sides of the conflict were using child soldiers and were engaged in illegal mineral trading.
Rwandan authorities have been "complicit in the recruitment of soldiers, including children, have facilitated the supply of military equipment and have sent officers and units" to help Nkunda, the report says.
In turn, Nkunda uses Rwanda as "a rear base for fundraising meetings and bank accounts," the report said.
The panel also said that Congolese troops have collaborated with Hutu rebels of the Democratic Forces for the Liberation of Rwanda (FDLR) and Mai Mai tribal fighters in the Congolese Resistance Patriots group.
"The group has identified at least three Congolese army commanders who are guilty of providing support" by supplying ammunition and taking part in joint operations against the Congolese army, said Jason Stearns, a former US analyst for the International Crisis Group, which led the investigation.
"While this collaboration is widespread and regular, the group [of investigators] has not been able to prove to what extent the top leadership of the army is involved in this practice. But it's clear that they know, and have done nothing to bring it to an end," he said.
Members of the FDLR fled to the hills of eastern Congo after being chased out of neighboring Rwanda in the wake of the 1994 Rwandan genocide, in which about 500,000 ethnic Tutsis were killed.
The presence in DR Congo of the FDLR, which has been accused by the UN of taking a leading role in the Rwanda killings, has led to the formation of opposition forces such as Nkunda's National Committee for the Defence of the People (CNDP).
The UN panel said that most of the weapons and ammunition used by DR Congo's armed groups have been sourced from the country's army and not the international arms market.
DR Congo government forces and CNDP fighters have been battling each other in the east of the country since August.
More than 250,000 people have fled their homes to escape the fighting.
Government soldiers and Nkunda's men have both been accused of atrocities against civilians. UN peacekeepers have had little impact in putting a stop to the violence.
The panel's conclusions were reached after they examined evidence which included confidential documents such as telephone records, bank statements, and flight records.
The report was also drawn from eyewitness testimony that was corroborated by at least three independent and reliable sources.
The other panel members are Dinesh Mahtani, a British finance expert; Mouctar Kokouma Diallo, a Guinea customs expert; Peter Danssaert, a weapons trafficking expert at a Belgium-based think tank; and Sergio Finardi, a military logistics expert for a group that monitors arms deals.