The fighting between government troops and M23 rebels threatens to spill over the border into Rwanda, where U.N. officials have accused government forces of supporting the rebels, Voice of America reported Saturday.
Rwanda's U.N. Ambassador denied the charge.
The U.S. State Department said a mortar shell lobbed by one of the sides in the Congo landed in Rwandan territory, threatening to turn the internal dispute into a larger regional conflict.
"We are ... troubled by reports that at least one round landed across the border in Rwanda," State Department spokeswoman Jen Psaki said in a statement. "This fighting puts at risk the fragile peace negotiations in Kampala and risks undermining the concerted efforts earlier this week to reach a final agreement and peacefully resolve the conflict."
Voice of America said thousands of refugees in the area have fled the DRC for Rwanda to escape the fighting.
The State Department said there have been reports of casualties but specific numbers of those killed or injured were not available.
The fighting comes two weeks after the two sides in the conflict agreed in principle to a peaceful end to the conflict and the eventual disarmament of the M23 fighters.