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, a United Nations report showed.

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 advanced weapons systems used by the rebels.

Between 3,000 and 4,000 RDF forces were active in Congo when the experts finished the reporting in April, according to their “conservative estimate.”

The Rwandan army’s support for the M23 is “sanctionable” by the UN, the experts said. “RDF’s de facto control and direction over M23 operations also renders Rwanda liable for the actions of M23.”

Rwanda has repeatedly denied backing the M23 rebels, who say they are fighting for the rights and protection of the Tutsi community in Congo.

The country accuses Congo of harboring rebels with links to the perpetrators of Rwanda’s 1994 genocide that killed more than 800,000 Tutsis and moderate Hutus. Violence has plagued resource-rich eastern Congo since then and more than 100 armed groups are active in the region. Both Uganda and Rwanda have regularly intervened directly or through proxy groups to protect their economic and security interests over the past three decades.

The current fighting has led to an “unprecedented humanitarian crisis” that has killed thousands and displaced millions in the past year, according to the report.

Elements of Congo’s own army and allied militias, known as Wazalendo, or “patriots” in Swahili, participated in the atrocities, including the targeting of UN peacekeepers, it stated.

“The recognition of Wazalendo by the government and the general public as ‘patriots’ fighting for liberation of the country gave these armed groups the excuse to act with impunity, with state security services either unable or unwilling to arrest perpetrators,” the experts said.

“The DRC has all the power to deescalate the situation if they want to, but until then Rwanda will continue to defend itself,” Rwandan government spokesperson Yolande Makolo told Bloomberg Monday. Congolese President Felix Tshisekedi has said Rwanda’s government and President Paul Kagame are “his enemy” and has consistently threatened to declare war on the country, she said.

Congo’s government said it would respond on Monday after the report was published.

Ugandan military intelligence officers are also coordinating with M23 leaders, providing them with logistics and transport, according to the report. M23’s main base in Congo is along Uganda’s border, “and M23 and RDF troops, vehicles, and military supplies regularly transit through Ugandan territory,” the group said.

“We have a very cordial relationship with the DRC government,” said Felix Kulayigye, the Uganda People’s Defence Forces spokesman. “We support a peaceful resolution of the conflict in DRC; does that mean we support one side? Uganda believes in regional initiatives for the resolution of the conflict.”