A group of armed attackers killed 11 people and injured 12 at a soccer sports field after a match in the Mexican city of Salamanca, its mayor, Cesar Prieto, said on Facebook on Sunday, but a motive was not immediately clear.

A woman and a child were among those injured in the “regrettable and cowardly” attack in the Loma de Flores community during a social gathering, Prieto added, describing the incident as a severe social breakdown.

"This incident adds to a wave of violence that we are sadly experiencing in the state, and particularly in Salamanca," he added. "Unfortunately, criminal groups are trying to subjugate the authorities, which they will not achieve."

An investigation has been launched into the attack, said the office of the attorney general in the state of Guanajuato, where Salamanca is located.

The office was coordinating efforts with municipal, state and federal authorities to strengthen security in the area, protect its people and find the likely perpetrators, it added in a statement.

"Those responsible will be found," Prieto added in his Facebook remarks.

Guanajuato is one of the most dangerous states in Mexico.