The Democratic Republic of Congo has asked for the extradition of rebel leader Gen Laurent Nkunda after his capture by neighbouring Rwanda.
Gen Nkunda, who has led an ethnic Tutsi rebellion in the east since 2004, is wanted for atrocities allegedly committed by forces under his command.
His arrest has been welcomed by the UN envoy to DR Congo.
The UN’s refugee agency has expressed alarm at moves to disarm a different rebel group – of ethnic Hutus.
en Nkunda was arrested in Rwanda, after fleeing attempts to arrest him in eastern DR Congo by a joint military force from both countries.
Correspondents say that it would appear that intense diplomacy has led to a deal under which DR Congo is letting Rwanda take action against its enemies – the Hutu rebels based in DR Congo – in return for taking out Gen Nkunda.
If both groups are neutralised, peace in the region would become a realistic prospect.
Some 4,000 Rwandan troops have entered DR Congo this week.
The BBC’s Thomas Fessy reports from the key east DR Congolese city of Goma that there is a sense of relief among residents there, who feel the war is nearing an end.
Rwandan army spokesperson Major Jules Rutaremara told the BBC that General Nkunda was being held by Rwandan forces in Rubavu district in western Rwanda, close to the border with the DR Congo.
DR Congolese Information Minister Lambert Mende confirmed his country wanted to extradite the rebel leader.
“He is Congolese,” said. “He committed his crimes in Congo.”
Rwanda has not yet said whether it will hand over its former ally.
Human rights group have accused Gen Nkunda’s National Congress for the Defence of the People (CNDP) – and also government forces – of numerous killings, rapes and torture.
Some 250,000 people fled their homes in North Kivu province when Gen Nkunda led an offensive towards the end of last year.
The UN recently accused Rwanda of backing Gen Nkunda.
A Tutsi like Rwanda’s leaders, he had guarded their western flank against attacks from the Hutu forces who fled there after the Rwandan genocide of 1994.
But in mid-November Rwanda shifted its position, announcing it would work with the Congolese to destroy the Hutu rebels.
The UN envoy, Nigerian former President Olusegun Obasanjo, told the BBC he welcomed Gen Nkunda’s arrest but said more work had to be done to end all conflict in the region.
“The problem of DRC is that you end something and then another ugly atmosphere rears its ugly head,” he said.
“This time we need to uproot everything, root, trunk and stem. And that is why our own mediation effort should not come to end, because our responsibility goes beyond even the eastern DRC, it covers the whole of the Great Lakes region.”
In a statement, the UN refugee agency (UNHCR) said it viewed “with some concern” a military build-up in North Kivu.
The joint Congolese and Rwandan operation is now expected to try to forcibly disarm the Democratic Forces for the Liberation of Rwanda (FDLR), an ethnic Hutu militia, some of whose leaders are accused of involvement in Rwanda’s 1994 genocide.
“Drawing from our past experience in this region, we fear that these operations could create new and massive displacement of the civilian population,” the UNHCR said.
It pointed out that there were already about 850,000 internally displaced people in the province.