The Ugandan army has provided support to the M23 rebel group operating in eastern Democratic Republic of Congo, a United Nations report seen by Reuters on Monday said, as escalating clashes there fuel fears of a new all-out conflict.

Uganda denied involvement, saying it is cooperating closely with the Congolese government forces. The UN has long accused Rwanda of backing the M23, which has repeatedly seized large parts of mineral-rich eastern Congo, allegations Rwanda denied.

Congo has been riven by conflict for decades. Uganda and Rwanda invaded in 1996 and 1998 for what they said was defence against local militia groups. Uganda is still conducting joint operations with Congolese troops against a rebel Ugandan group.

The Tutsi-led M23 rebels have been waging a fresh insurgency in Congo's militia-plagued east since 2022.

Ugandan troops were part of a regional force deployed in November 2022 to monitor a ceasefire with the M23. Congolese authorities called for the force to withdraw last year, saying it was ineffective.

"Since the resurgence of the M23 crisis, Uganda has not prevented the presence of M23 and Rwanda Defence Forces (RDF) troops on its territory or passage through it," the UN Security Council's Group of Experts said in the report, which was sent to the U.N. Security Council Sanctions Committee at the end of April and then to members of the Security Council in June.

The UN group also said it had obtained evidence confirming active support for M23 by officials from the military and military intelligence, with M23 leaders, including the sanctioned Sultani Makenga, travelling to Uganda for meetings.

Contacted by Reuters, deputy spokesperson for Uganda's armed forces, Deo Akiiki, said such reports falsely accuse the east African country's army when its relationship with the Congolese forces (FARDC) is at its best.

"It would be mad for us to destabilise the same area we are sacrificing it all to have it stable," Akiiki said.

The UN report said some 3 000-4 000 Rwandan soldiers were fighting the Congolese army alongside the M23. The Rwandan army's "de facto control and direction over M23 operations also renders Rwanda liable for the actions of M23", the experts said.

In response, Rwanda said Congo was financing and fighting alongside a Hutu rebel group, the Democratic Forces for the Liberation of Rwanda (FDLR), that has attacked Tutsis in both countries.

"The DRC has all the power to deescalate the situation if they want to, but until then Rwanda will continue to defend itself," Rwanda government spokesperson Yolande Makolo told Reuters.