Rwanda has been accused of backing a new rebellion in eastern Congo that has forced thousands of civilians to flee and created fears of a fresh humanitarian crisis in the region.
The conflict pits army mutineers against government forces from the Democratic Republic of Congo (DRC) and threatens to pull apart a fragile peace deal between Rwanda and its much larger neighbour.
"This weekend was very violent, the most intense fighting so far," said Emmanuel de Merode, the chief warden of DRC's Virunga Park where much of the conflict is taking place. "There was heavy shelling all Saturday and much of Sunday." Already at least 7,000 refugees have fled into Rwanda and a further 14,000 into Uganda to escape the fighting.
Rwanda has strongly denied sending arms and recruits to back a mutiny in the DRC army that began last month and was led by former Tutsi rebels who have previously been linked with the government in Kigali. But United Nations internal reports, seen by the BBC, claimed that 11 rebel soldiers who were interviewed said they were Rwandan nationals and had been recruited there and sent across the border into the DRC to fight.
A similar crisis four years ago prompted the UK and France to intervene in an effort to get Rwanda to use its influence to halt the fighting.
Eastern Congo still suffers the after-effects of the Hutu-led genocide in Rwanda that left 800,000 people dead, many of them ethnic Tutsis, and sent refugees pouring across the border in 1994. Among the refugees were Hutu forces fresh from the massacres who formed a rebel group calling itself the FDLR. Rwanda has repeatedly accused the DRC of harbouring these genocidaires and sent its own forces into eastern Congo to pursue them. The Tutsi-led government in Kigali has also been accused in UN reports of backing proxy groups inside the DRC.
Four years ago a Tutsi-led Congolese rebel army, the CNDP, threatened to overrun the region's main aid hub in the city of Goma, sparking an evacuation of international staff. That crisis was resolved after peace talks between Kigali and the government of Joseph Kabila in Kinshasa which called for the CNDP to be integrated into the Congolese army.
The Congolese government has been under pressure to hand over former CNDP leader Bosco Ntaganda, who is wanted for war crimes by the International Criminal Court, but had been made an army officer under the 2009 peace deal.
Ntaganda, who is known as "the Terminator", launched the April mutiny amid signs that the government was considering surrendering him to the ICC. Other former rebel commanders, unhappy with the army integration process, joined him to establish a force calling itself "M23", thought to number up to 800 soldiers. They have since moved into the remote hills near the border with Rwanda and Uganda.