The Democratic Republic of Congo has accused Rwanda of sending troops across the border, and threatening the eastern city of Goma.
The local provincial governor said Rwandan soldiers backing the Congolese rebel leader Laurent Nkunda had crossed the border three days ago.
Rwanda has denied that any of its troops are inside DR Congo.
Rwanda twice invaded its neighbour in the 1990s and has accused the government of backing Rwandan rebels.
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.
Earlier, Gen Nkunda's rebels said they had captured an army base at the border village of Ramangabo near Goma after a day of intense fighting with government forces.
UN peacekeepers said heavy fighting had occurred around the base, but could not confirm that it had fallen.
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.
Until now, Gen Nkunda has always said he was only protecting his Tutsi community from attacks by Rwandan Hutu rebels.
Some of these are accused of carrying out the 1994 genocide of Tutsis and moderate Hutus in Rwanda.