Thirteen people were killed in three separate firefights in Mexico in the northeastern city of Matamoros, Tamaulipas, authorities said Sunday. State officials said that all 13 of the deceased were "armed civilians", according to the Mexican newspaper El Universal.

Matamoros is located on the Rio Grande River across from Brownsville in the US state of Texas and rival gangs have battled over its drug-smuggling routes for years, according to AFP.

The first of the three shootouts occurred at noon on Sunday along the Matamoros-Reynosa highway, leaving four men and one woman dead. AFP reports that the shootout was between two groups of civilians.

About four hours later, along the same highway, Mexican Marines reportedly clashed with "armed civilians" in several vehicles. Following the shootout, authorities found four men dead in an armored Cadillac Escalade loaded with weapons and ammunition.

Minutes later, Marines killed four more men identified as armed civilians in a third shootout on a Matamoros street.

None of the deceased have been identified, and no ties to organized crime have been announced. However, the state of Tamaulipas is known for raging cartel violence and is host to the notorious Zetas gang.

Under current Mexican President Enrique Peña Nieto, more than 6,000 have died as a result of drug-gang violence. Approximately 70,000 died in Mexico's drug war under Nieto's predecessor, President Felipe Calderon.

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