Rwanda has previously denied backing Gen Laurent Nkunda's rebels
Rwanda and the Democratic Republic of Congo have both been directly helping rebels fighting in eastern DR Congo, according to a draft report for the UN.
Rwanda is accused of supplying aid and child soldiers to Tutsi rebels. Rwanda has denied such accusations previously.
The report, seen by the BBC, also says the Congolese army collaborates with the Rwandan-Hutu militia, the FLDR.
The final report is being presented to the Sanctions Committee of the UN Security Council in the next few days.
The BBC has seen a draft copy of the findings by UN-appointed experts on alleged violations of the arms embargo in DR Congo.
It alleges that both Rwanda and DR Congo have used rebel movements as proxies in a covert struggle.
Dissident rebel leader General Laurent Nkunda says he is protecting his Congolese Tutsi community from attack by leaders of the Rwanda genocide, but critics say the conflict is about raw power and control of mineral resources.
Recent fighting in the east has displaced some 250,000 people since August. Both government and rebel forces have been accused of raping, mutilating and killing civilians.
The draft UN report alleges the Rwandan authorities have supplied Gen Nkunda's forces with military equipment, the use of Rwandan banks, and allowed the rebels to launch attacks from Rwandan territory on the Congolese army.
The report claims rebels have helped fund the conflict by exploiting mines
Perhaps most damagingly, it claims Rwandan officers brought recruits - some of them child soldiers - up to the border, on behalf of the rebels.
The report also says a barrage of artillery and mortar fire - which helped Gen Nkunda's forces advance in October on the North Kivu province capital of Goma - came from Rwandan territory.
Rwanda's ambassador to the UK, Claver Gatete, denied to the BBC that the fire had come from Rwanda.
He also said that neither he nor the Rwandan government had seen the draft report and did not know its contents.
Mr Gatete added that records of satellite phone calls between the office of Rwanda's President Paul Kagame and Laurent Nkunda's CNDP rebels, mentioned in the report, would need to be verified.
The BBC's Mark Doyle, who has recently been in DR Congo covering the conflict, says the draft UN report does not mean Gen Nkunda's rebels are controlled from Rwanda.
But the fact that the UN has been monitoring phone calls between the Rwandan presidency and Gen Nkunda will infuriate President Kagame and further poison the relationship between the UN and Rwanda.
'Killing our people'
The UN experts also gathered information suggesting Congolese army support for the FDLR, some of whose leaders were involved in the Rwandan genocide 14 years ago.
This militia raised millions of dollars to fund their war through the illegal trade in minerals, says the BBC's Thomas Fessy in Kinshasa. The Congolese government has previously denied working with the FDLR to exploit the region's rich mines.
And its Information Minister Lambert Mende Omalanga said the latest allegations were totally untrue.
"Those people are still killing our people - Congolese people - they are really destroying our environment there," he told the BBC.
"We cannot ever support them and accept whatever they are doing in our country."
The UN report names the foreign companies that traded with the FDLR, and recommends that sanctions be imposed against them and individuals named in the report.
This is now for the UN Security Council to decide.