At least two people have been injured after a grenade exploded outside a televison station in northern Mexico.

According to local authorities, the attack occurred outside the offices of the Televisa network, in the city of Monterrey on Sunday.

It was the third attack in the past three months against an affiliate of Televisa, which is Mexico's leading television network.

A police source in the state of Nuevo Leon told AFP news agency that the blast shattered some of the building's windows.

"The explosives were thrown under a parked vehicle and also caused damage to another vehicle owned by the network," the source said.

The city of Monterrey is at the heart of fighting between the Zetas and Golfo drug cartels.

Reporters targeted

The Inter-American Press Association lists Mexico as the Latin America's most dangerous country for journalists, with nine murdered this year.

Journalists are often caught in the cross fire as armed drug cartel enforcers battle for control of lucrative drug smuggling routes into the United States, and the Mexican military and police attempt to crack down on them.

Around 26,000 people have been killed in drug-related violence in Mexico since 2006, when president Felipe Calderon launched a military crackdown on violence across the country.

Some 7,000 people have been murdered since the start of the year.

Last week, hundreds of media workers marched in protest in major Mexican cities to protest attacks on the press and the recent abduction of four reporters, who were eventually released.