Six police officers were killed Sunday in northern Mexico after being attacked by armed men traveling in armored vans, authorities said in the state of Nuevo Leon, plagued by recurring violence linked to drug trafficking.

The clashes erupted in the middle of the night when agents of the Civil Force were targeted by heavily armed unknown persons aboard a dozen armored vans, "and who outnumbered our colleagues", indicated the Secretary of State for Security in a statement posted on Facebook.

Six police officers were killed and four injured, adds the press release, which does not specify whether the assailants belonged to one of the criminal organizations that operate in this border region of the United States.

Mexico has been hit by a spiral of violence that has claimed some 340,000 lives -- most attributed to organized crime -- since 2006, when a controversial military counter-narcotics offensive was deployed.