Security forces are leading Mexico's war on drug cartels as the toll from four years of violence reaches 28,000

Mexican troops have clashed in the northern state of Tamaulipas with armed men believed to belong to the Los Zetas drug cartel, killing 11 of them, the defence ministry says.

The firefight occurred on Wednesday night when a patrol responded to reports of armed men in the town of Nueva Ciudad Guerrero, the ministry said in a statement.

It said the assailants fired on the patrol, triggering the gun battle.

"Eleven presumed aggressors died," the ministry said.

It said two others who were taken into custody said they belonged to Los Zetas, a group formed in the 1990s by former members of an elite military unit.

After the clash, security forces recovered nine rifles, four handguns, a grenade launcher and bullet-proof vests, the ministry said.

An estimated 28,000 people have been killed in drug-related violence over the past four years in Mexico as cartels have fought among themselves for control of drug routes, and the government has unleashed the military.

Tamaulipas, one of the states most affected by the violence, is where 72 illegal immigrants were massacred in August, allegedly by Los Zetas.

In a separate incident, Salvador Mikel Rivera, the attorney general of Veracruz state, has confirmed that assailants killed a Colombian businessman working for an multinational oil services company in eastern Mexico on Tuesday.

Francisco Ruiz Palacios, 36, was shot several times after exchanging words with his attackers as he was exiting a car near the city of Poza Rica, on the northern Gulf of Mexico, Rivera said on Thursday.

Ruiz had worked for Weatherford, which is contracted by Mexico's state oil company Pemex to work on the extraction of crude oil.

Source: Agencies