Rwanda recruited and trained refugees from Burundi, among them children, whose ultimate goal was to remove President Pierre Nkurunziza from power, UN experts told the Security Council.
The panel said in a confidential report obtained by AFP on Thursday that they had spoken to 18 Burundian refugees who provided details of their military training last summer in a Rwandan forest camp.
"They reported that their ultimate goal was to remove Burundian President Nkurunziza from power," said the report by the panel of experts for the Democratic Republic of Congo.
Burundi has repeatedly accused Rwanda of backing rebels intent on overthrowing the government in Bujumbura, allegations Rwanda denies.
The refugees, who had crossed into the DR Congo, told the experts that they had been recruited in the Mahama refugee camp in eastern Rwanda in May and June 2015.
The group was given two months of military training in Rwanda by instructors, some of whom were Rwandan military personnel, the report said.
"Their training included military tactics and the maintenance and use of assault rifles and machine guns, as well as ideological and morale-building sessions," it added.
Some were also trained in the use of grenades, anti-personnel and anti-tank mines, mortars and rocket-propelled grenades.
Six of the 18 trained combatants were minors.
The refugees told the experts that at least four companies of 100 recruits were being trained at the camp and that they were transported around Rwanda in military trucks, often with Rwandan military escorts.
The Burundian combatants showed the experts fake identification cards from the DR Congo that were produced in Rwanda.
The US-based advocacy group Refugees International said last month that Burundian men and boys were being recruited from the Mahama camp and facing threats if they refused.
"I haven't even seen the tiniest evidence of that, so it becomes a lot of politicking," Rwandan President Paul Kagame said in December, calling the accusations "childish".
UN experts also interviewed six Rwandan and Congolese nationals arrested on suspicion of arms smuggling at the Congolese-Rwandan border in October and November last year.
Some of the suspected arms smugglers told the UN experts that the "weapons were to be used in support of an armed group in Burundi," said the report.
Burundi has been in turmoil since Nkurunziza announced plans in April to run for a third term, which he went on to win.
More than 400 people have died since then and at least 230,000 have fled the country.
During a visit to Burundi last month, UN Security Council ambassadors met Nkurunziza, who again accused Rwanda of backing rebels.
The African Union has proposed sending military observers to the Rwanda-Burundi border.