The head of the US war crimes office has warned Rwanda’s leaders, including President Paul Kagame, that they could face prosecution at the International Criminal Court for arming groups responsible for atrocities in the Democratic Republic of Congo.

Stephen Rapp, who leads the US Office of Global Criminal Justice, told the Guardian that the Rwandan leadership may be open to charges of “aiding and abetting” crimes against humanity in a neighbouring country – actions similar to those for which the former Liberian president Charles Taylor was jailed for 50 years by an international court in May.

Rapp’s warning follows a damning United Nations report on recent Rwandan military support for M23, an insurgent group that has driven hundreds of thousands of people from their homes since April as it seized territory in the eastern DR Congo.

The group is led by Bosco Ntaganda, known as the Terminator, who was indicted by the international criminal court six years ago for war crimes including the forced recruitment of child soldiers. The UN report accuses Rwanda of shielding Ntaganda from justice.

On Saturday, Washington said it would halt some military aid to Rwanda after the UN report. The aid freeze and Rapp’s public intervention mark a significant shift away from once solid US support for Mr. Kagame, which was rooted in lingering guilt over international inaction during the 1994 genocide.

Rapp, who previously served as chief prosecutor at the Rwanda genocide tribunal and later initiated the prosecution of Taylor over his crimes as president of Liberia in supporting rebels in neighbouring Sierra Leone, said Rwandan support for M23 and other armed groups “has to stop” because it “maintains the lawlessness and at the end of the day enables the ongoing commission of atrocities”.

“There is a line that one can cross under international law where you can be held responsible for aiding a group in a way that makes possible their commission of atrocities,” he said.

Rapp said the evidence by the UN group of experts of Rwandan government support for M23 and other armed groups, including sending weapons and troops into the DR Congo, exposed Kagame and other senior officials to investigation for war crimes.

The UN report by a group of experts appointed by the security council said it had “found substantial evidence attesting to support from Rwandan officials to armed groups operating in the eastern DR Congo”, including shipping weapons and money to M23 in breach of a UN arms embargo and other sanctions.

It also offers evidence of “direct Rwandan Defence Forces (RDF) interventions into Congolese territory to reinforce M23” and “support to several other armed groups”.

Rwanda’s foreign minister, Louise Mushikiwabo, described the UN report as “bad information”. “As we have made clear from the outset, Rwanda is neither the cause nor the enabler of instability in the eastern DRC,” she said.

Rwandan officials are expected to meet the UN group of experts in Kigali this week. “We will go through each allegation contained in the interim report and debunk them line by line,” she said.

Rwanda’s invasion of what was then Zaire in 1996 to clear out refugee camps sheltering armed groups responsible for the genocide two years earlier was quietly backed by Washington and London, which did not challenge a second invasion two years later and Kigali’s attempt to create a proxy administration in eastern DR Congo using loyal armed groups.