Senior Congolese officials charged that Rwandan President Paul Kagame had made good on his threat to invade, claiming Rwandan troops had crossed into eastern Congo and were clashing with militias there.
UN officials said they were investigating the unverified claims -- allegations that came as Kagame told his country's parliament that Rwandan troops "might" already be in Congo, pursuing Rwandan rebels based there.
Congo's government protested, while a Congolese Cabinet minister on the scene in the east, Mbusa Nyamwisi, said: "We are on a war footing."
Speaking Tuesday from the eastern town of Beni, he claimed there was fighting nearby -- "We are being attacked by the Rwandan troops," he said.
Lawless eastern Congo was the scene of the worst fighting in a devastating 1998 to 2002 central African war, and remains home to numerous, vying militias. The region is the site of frequent clashes, which residents frequently blame on Rwanda and its one-time Congolese rebels allies.
On Tuesday, local officials, Congolese commanders, priests and other community leaders told UN officials and journalists that Rwandan forces were in the area.
Villagers reaching Beni told authorities that communities north of Goma, near Congo's border with Rwanda, had been attacked, with at least three villages burned, Nyamwisi said. The displaced reported 15 people killed at one village, Ikobo, he said.
Nyamwisi claimed two brigades of Rwandan troops were fighting alongside former rebel forces that had been allied to Rwanda during the 1998 to 2002 war. He gave no evidence.
Congolese "intelligence services" also reported Rwandan troops north of Goma, the regional military commander, Colonel Etienne Bindu, said.
Rwanda invaded Congo in 1996 and 1998 to pursue Rwandan Hutu forces responsible for Rwanda's 1994 genocide, when extremists from Rwanda's Hutu majority orchestrated the slaughter of more than 500,000 minority Tutsis and moderate Hutus. The 1998 invasion sparked a five-year war that divided resource-rich Congo, drawing in the armies of six countries and killing an estimated 3.2 million people.
A series of peace deals and power-sharing accords led to the withdrawal of foreign armies and an end to major fighting in 2002. But sporadic fighting persists in the east, which was controlled by Rwanda during the war.
Rwanda -- which itself was unable to fully quell the Rwandan rebels in its five years of wartime control of east Congo -- accuses a 5-month-old UN-led disarmament campaign of failing to neutralize the estimated 10,000 Rwandan Hutu fighters still in the east.
"Anytime the United Nations ignores or fails to deal with the problem of [the rebels], we shall do it ourselves, and this will not take long, or, we might even be doing it now," Kagame told lawmakers in Kigali, the Rwandan capital.
In Kinshasa, Congo's capital, Congo Foreign Affairs Minister Raymond Ramazani Baya said Kagame's threat itself violated regional peace deals.
"The Congo government rejects this pretext, and states that it knows of nothing that justifies a new attack," Baya said.
The UN humanitarian agency said it would send an investigating mission Wednesday to an area near the Rwandan border where Rwandan troops were reportedly fighting Congo government-allied Mai Mai ethnic militias, said Bernard Leflaive, the agency's director in Goma.