The experts said there was "substantial evidence" of direct Rwandan army intervention and of Rwandan support for the region's M23 rebels, in such forms as weapons, ammunition and uniforms.
According to the UN experts' report, Rwanda's military intervened to "reinforce" the M23 as well as to combat the FDLR -- a descendant of Rwandan Hutu extremist groups that carried out the 1994 Tutsi genocide in Rwanda.
Rwanda provided troop reinforcements to the M23 "for specific operations, in particular when these were aimed at seizing strategic towns and areas," the report added.
Rwandan troops also led joint attacks with M23 fighters against Congolese positions in May, according to the report.
The 236-page document for the UN Security Council is expected to be published in the coming days.
The report comes as the Democratic Republic of Congo has repeatedly accused Rwanda of backing the M23.
The militia has captured swaths of territory in the DRC's restive east since it emerged from dormancy late last year.
Rwanda has repeatedly denied that it supports the rebels, but the United States and France, among other Western countries, have agreed with the DRC's assessment.