KINSHASA (Reuters) - Democratic Republic of Congo on Wednesday demanded targeted sanctions against Rwandan and Ugandan officials accused by a U.N. experts panel of backing a six-month-old insurgency in its volatile eastern borderlands.

The U.N. Security Council's Group of Experts said in a confidential report seen by Reuters that both Rwanda and Uganda were supporting the M23 rebels, who are expanding their control of parts of Congo's mineral-rich North Kivu province.

The experts, who monitor compliance with U.N. sanctions and an arms embargo on Congo, singled out Rwandan Defense Minister General James Kabarebe as heading M23's de facto chain of command.

They also said that senior Ugandan officials had provided M23 with "troop reinforcements ... weapons deliveries, technical assistance, joint planning, political advice and facilitation of external relations."

Senior Rwandan diplomat Olivier Nduhungirehe said the U.N. experts had "been allowed to pursue a political agenda".

Congo's government spokesman Lambert Mende said on Wednesday that those named in the report as backing the rebellion should be placed under U.N. sanctions.

"It's more important than ever, as now we have proof that the drama in North Kivu is being manipulated by criminals who hold positions of power," he told Reuters.

"We've taken note of this report which confirms what we already know about Rwanda and contains new information about Uganda ... We're in contact with our neighbors in Uganda over these very serious allegations," he said.

While Kinshasa has regularly accused Kigali of meddling in Congo since an interim report in June revealed Rwandan links to the rebels, Uganda has recently played a central role in attempts to broker a negotiated settlement to the conflict.

Uganda's deputy minister for foreign affairs rejected the report on Wednesday, saying allegations the country was supporting M23 were "rubbish, rubbish, rubbish".

"The leaking of the report by the Panel of "Experts" can only be in bad faith and intended to cause mischief and undermine the regional efforts to find a durable solution to the conflict in Eastern DRC," Okello Oryem told reporters at a news conference in Kampala.

Oryem added Uganda had been denied an opportunity to respond to the "fabricated allegations" through the U.N. Security Council's established procedures.

Nearly half a million people have been displaced due to the fighting between M23 and the Congolese army this year.

Rwanda twice invaded its much larger western neighbor in the late 1990s in what it said was the pursuit of Hutu fighters responsible for a 1994 genocide that killed around 800,000 Rwandan Tutsis and moderate Hutus.

Uganda also sent troops across the border during a 1998-2003 war in the region that left millions dead.

(Additional reporting by Elias Biryabarema in Kampala; Reporting by Jonny Hogg; Writing by Joe Bavier; Editing by Jon Hemming)