Thousands of Rwandan troops are backing a rebellion in eastern Democratic Republic of Congo that has displaced millions of people and brought the two countries to the brink of war, United Nations experts said.

In a report seen by Bloomberg News set to be published Monday, the experts said they had “unequivocal” proof of Rwandan support of the M23 militia, including video and drone surveillance of Rwandan Defense Forces and evidence of advance weapons systems used by the rebels.