At least 25 people died on Thursday in a clash between Mexican soldiers and gunmen in the northern state of Nuevo Leon, a military spokesman said.
It appears all the fatalities were gunmen, he said.
The gun battle occurred 15 km east of the municipality of General Treviño near the border with the neighbouring state of Tamaulipas.
The fighting erupted when soldiers were fired upon when they approached a camp full of armed men, Mexico City daily La Jornada said on its website.
Nuevo Leon, which shares a short stretch of border with the US state of Texas, since March has been experiencing a resurgence in the violence resulting from the ongoing "war" between the rival Los Zetas and Gulf drug cartel.
The state capital, Monterrey - Mexico's wealthiest city - has been greatly affected by the violence.