Congo residents and M23 rebels say that another rebel group, the FDLR, has crossed the border into Rwanda and is fighting the Rwandan army.
An M23 officer who would only give his name as Lt. Paluku and people who live in the Congolese village of Kibumba say they have heard fighting at the border since 5 a.m.
Hours later AP journalists were fired at by FDLR soldiers retreating from the border. No reporters were injured.
The Democratic Force for the Liberation of Rwanda, known by its French acronym FDLR, is a group of Hutu extremists, many of whom are blamed for leading Rwanda's 1994 genocide of Tutsis. FDLR members fled to eastern Congo after the massacres. The FDLR has fought the Rwandan army, the Congo army and other militias.