Officials within the Rwandan military have provided up to 300 fighters as well as weapons and ammunition to rebels battling government forces in neighbouring Democratic Republic of Congo, Human Rights Watch said on Monday.
Rwanda denies backing the rebellion in the mountainous east of Congo, which has for the last two months pitted the army against troops loyal to Bosco Ntaganda, a renegade Congo general wanted by the International Criminal Court (ICC).
Kigali has a history of supporting armed groups in Congo, citing a need to pursue Rwandan rebel fighters linked to the country's 1994 genocide, but has also faced accusations of contributing to the ongoing violence and pillaging the region's vast mineral wealth.
It has also provided assault rifles, anti-aircraft artillery and ammunition to the rebels who are currently holed up on three hills just a few kilometres from the Rwandan border, the rights group said in a report.
"The role played by some Rwandan military officials in supporting and harbouring an ICC war crimes suspect can't just be swept under the rug," Anneke Van Woudenberg, HRW's senior Africa researcher, said.
Congolese military officials have estimated that the rebels reached their current position in Rutshuru territory, North Kivu province, with around 300 fighters, meaning that if HRW's claims are correct, Rwandan recruits would have effectively doubled the size of their force.
Ntaganda was spotted talking to a Rwandan military officer by witnesses in the Rwandan town of Kinigi on May 26, according to HRW, which also reported that at least six rebels had been executed after trying to desert.
" Rwanda should immediately stop all support to Ntaganda and assist in his arrest," Van Woudenberg said.
Rwanda's foreign minister last week accused Human Rights Watch and other non-state actors of trying to destabilise the region and of spreading false rumours over its involvement in the rebellion.
The government of Democratic Republic of Congo has said it is jointly investigating the claims with Rwanda, which risk seriously damaging relations between the two countries.