Thirteen people were killed in shootouts on Sunday around the northeastern Mexican city of Matamoros in one of the worst recent outbreaks of violence in an area ravaged by drug gangs.
Three gunfights took place around the city opposite Brownsville, Texas, two of which were exchanges between gunmen and Mexico's armed forces, according to a statement from the state government of Tamaulipas.
Eight men died in the fighting with Mexican Marines after four men and one woman were killed in an earlier clash between unidentified armed groups, the state government said.
None of the dead have yet been identified.
Seeking control of smuggling routes into the United States, drug cartels have in the past few years been responsible for a slew of massacres, gunfights and kidnappings in Tamaulipas, giving the state the reputation as one of the most lawless in Mexico.
President Enrique Pena Nieto took office in December pledging to eradicate the gang violence that has claimed about 80,000 lives since the start of 2007, but parts of Mexico are still regularly racked by shootings and executions.