Fighting between rebels and government troops resumed in August
Forces loyal to rebel leader Laurent Nkunda have taken a major army base in eastern Democratic Republic of Congo, UN peacekeepers have confirmed.
The UN peacekeeping force Monuc said rebels seized weapons and supplies after overrunning the strategically important camp of Rumangabo overnight.
The attack came as DR Congo accused Rwanda of sending troops across the border to threaten the city of Goma.
Rwanda denied the claim, saying it had no reason to attack its neighbour.
Rwanda twice invaded its neighbour in the 1990s and has accused the government of backing Rwandan Hutu rebels.
Military sources report several thousand uniformed troops massing just over the border in Rwanda.
The local provincial governor said Rwandan soldiers backing Gen Nkunda had crossed the border three days ago.
The Congolese ambassador to the United Nations Atoki Ileka said he would call for an urgent meeting of the UN Security Council if Goma was attacked.
"Rwanda is already in the DRC," he told the BBC's Network Africa programme.
"Rwanda, and I say Mr Paul Kagame, the president of Rwanda, are the spoilers in the region," he said.
"Laurent Nkunda in our view is some kind of a proxy for Rwanda."
But Rosemary Museminari, Rwanda's state minister for foreign affairs, said her country would have no reason to attack DR Congo.
"We are not amassing, we are not putting any special forces on the border with Congo," she said.
"I think the DRC is trying to divert attention from the real problem.
"The fact is that the Congolese army is finding it difficult in dealing with the rebel forces in their region."
She added that Rwanda had no interest in supporting Gen Nkunda, and simply wanted to secure its own borders against forces in DR Congo who had committed genocide in Rwanda and had been "creating havoc" in the region.
The UN mission in DR Congo, Monuc, was confirming earlier claims by Gen Nkunda's rebels that they had captured the major army base at the border village of Ramangabo near Goma after a day of intense fighting with government forces.
Last week, Gen Nkunda said he would take his fight across DR Congo.
Fighting resumed in August between his forces and the army, despite a peace deal signed in January.
More than 100,000 people have fled the clashes, aid workers say.
Monuc head Alan Doss described the situation in the east and especially in North Kivu province as "very serious".
"When these situations develop they often translate themselves into ethnic dimensions and that is very dangerous," he told the BBC's Focus on Africa programme.
Until now, Gen Nkunda has always said he was only protecting his Tutsi community from attacks by Rwandan Hutu rebels.
Rwanda believes that Hutu FDLR fighters - some of whom are accused of carrying out the 1994 Rwandan genocide against Tutsis and moderate Hutus - will use DR Congo as a base to destabilise Rwanda again.
Mr Doss said the FDLR rebels were also a threat to DR Congo itself.
"It's the same FDLR that is committing violence against the people of the Congo, in particular against women and children," he said.
"It's the same FDLR that is pillaging the country's natural resources and exporting them illicitly and illegally."