“The truth always triumphs in the end. We hope that conclusions will be drawn quickly to put an end to Rwanda's interference and bring back lasting peace,” he tweeted.
The M23 insurgency stems from the long fallout from the 1994 genocide in Rwanda. The group was formed in 2012 and claims to defend the interests of Congolese Tutsis, the ethnic group shared by Rwanda's president, Paul Kagame, against Hutu militias.
Since May M23 has waged its most sustained offensive in years, killing dozens and displacing tens of thousands of people. By July it controlled a territory almost three times as large as it did in March, the UN group said.
The M23's resurgence has inflamed regional tensions and spurred deadly protests against the UN peacekeeping mission in Congo, which civilians accuse of failing to protect them.
The UN group detailed evidence, including photos of Rwandan soldiers in an M23 camp, drone footage showing columns of hundreds of soldiers marching near the Rwandan border and photos and videos showing M23 fighters with new uniforms and equipment similar to that of the Rwandan army.
Rwandan troops and M23 jointly attacked the Congolese army camp in Rumangabo in May, it said. When M23 took control of the strategic border town of Bunagana in June, Rwandan soldiers were present or had provided equipment to the rebels, the report said.
Rwanda and neighbouring Uganda have a long history of military intervention inside Congo. The two countries invaded in 1996 and 1998, saying they were defending themselves against local militia groups.
A target of M23 and Rwandan operations in Congo has been the Democratic Forces for the Liberation of Rwanda (FDLR), a Hutu militia Rwanda accuses Congo of using as a proxy. Congo's government has denied this.
Some members of Congo's army have supported and fought alongside a coalition of armed groups, including the FDLR, the UN experts said.