More than 1,000 Rwandan soldiers crossed into the Democratic Republic of the Congo (DRC) offering "direct support" to rebels who captured a major city, according to sources quoted in a UN inquiry.

Uganda also helped the M23 militia with "logistics" as it fought a deadly and successful offensive against the Congolese army, the UN Group of Experts said in a letter to the security council.

The allegations deal a fresh blow to Rwanda after months of mounting evidence which, despite Kigali's point-blank denials, prompted Britain to suspend aid worth £21m last week.

The experts said Rwanda Defence Force (RDF) units operated alongside M23 fighters in the rebels' advance on the eastern commercial hub of Goma last month.

When M23 began its offensive on Goma "it benefited from direct RDF support during combat on the frontlines at the village of Kibumba, according to former RDF officers, FARDC [Congolese army] officers and local leaders," the experts stated.

The FARDC killed more than 40 rebels and RDF soldiers "many of whom wore RDF uniforms and carried weapons used by the RDF", they said. "When M23 progressed towards Goma on November 19, 2012, RDF units operated alongside M23 in combat at the airport and close to one of Goma's border posts into Rwanda."

Rwandan forces and rebels "together" took Goma, "marching through downtown dressed in a combination of RDF and new M23 uniforms".

Sources estimated that "well over" 1,000 RDF troops came from Rwanda to assist M23 in these operations, the letter said, and about 500 were inside Goma. On 20 November, the day Goma was captured, "a mixture of M23 and RDF troops clandestinely entered into Goma from the Rwandan town of Gisenyi through small streets situated between the town's two official border crossings".

Rwanda's General Emmanuel Ruvusha is said to have helped the M23 commander, Sultani Makenga, lead the attack. The experts were also informed that General Bosco Ntaganda – dubbed "The Terminator" and wanted by the international criminal court – led M23 troops operating near the Rwandan border at Kibumba and in the vicinity of Goma.

The rebels acquired camouflaged uniforms similar to those used by Rwandan forces. Former rebels told the experts "the aim of using nearly indistinguishable uniforms was to create confusion" when RDF units joined the rebels.

The document includes photos of several people it identifies as Rwandan troops on Congolese territory.

Uganda deployed troops near the Congolese border in late October to make sure M23 territory "was not left unprotected during the imminent offensive", the experts added, saying they witnessed the delivery of hundreds of boots to the rebels at the border town of Bunagana on 14 October.

The experts said their inquiry "strongly upholds" previous claims that neighbouring countries had provided significant backing to the rebels, who have been accused of widespread human rights abuses.

Rwanda, due to take up a seat on the UN security council next month, adamantly rejects previous allegations by the UN experts that it created, equipped, trained and directly commanded the M23 rebellion in Congo's North Kivu province. Accusations of Ugandan involvement have also been denied.

Louise Mushikiwabo, Rwanda's foreign minister, said on Tuesday: "Rwanda cannot continuously engage with factless accusations. We have serious issues to deal with. We don't need the distraction. We'll leave it to those who enjoy fiction."

Paul Kagame, the Rwandan president, tweeted: "The actual truth will alwz finally prevail. Pple need to support on-going efforts by ICGLR [International Conference on the Great Lakes Region] and stop blame-game#E.DRC."

The UN security council has passed a resolution that threatened action against "those providing external support" to M23, which draws most of its strength from Tutsi former rebels who had been integrated into Congo's national army but mutinied in April. It has already ordered sanctions against three M23 military commanders including Makenga.

Commentators said the UN allegations fit a pattern. Jason Stearns, a political analyst and former co-ordinator of the UN Group of Experts on Congo, said: "It's not a surprise as every major M23 offensive so far has received support from the Rwandan army. But it was very risky for Rwanda as it comes at the height of international criticism of its involvement in Congo."

Publicly, Kagame joined other leaders in urging M23 to pull out of Goma soon after it fell. Stearns said: "To an outsider it's ironic. They support M23 yet call for their withdrawal as well."

Rwanda would continue to deny meddling in its neighbour, he predicted, just as it did while secretly backing a rebel group there in 2009. "But as soon as a solution was available, they were lauded internationally for finding a constructive solution. That's the sort of magic they want to work here but they haven't found it yet."

Congolese soldiers regained control of Goma on Monday, though the rebels who occupied it for nearly two weeks continued to stake out positions just 3km away, threatening to move back in if the Congolese government fails to meet their demands. Officials said peace talks with the rebels would be held "in the next few days" in Uganda's capital, Kampala.

The UN humanitarian agency OCHA said at least 130,000 people have been displaced and are living in camps in and around Goma. UN officials said a camp about 10 miles outside Goma had been raided by unidentified gunmen late on Friday. Several women were raped, and food and supplies stolen.