The attack followed a similar incident at their studios in Matamoros, in the north-east, just hours earlier.

While the blasts caused some damage to buildings, no one was injured.

Human rights groups say Mexico is one of the world's most dangerous countries for reporters.

Last month, two Televisa journalists were kidnapped, along with two colleagues from other media, when they were covering corruption allegations at a prison in Durango state.

The kidnappers reportedly demanded that the TV stations the men worked for broadcast a series of videos accusing local officials of ties with a drug cartel.

Televisa cancelled one of its news programmes in protest at the kidnapping.

The host of the Starting Point show said the team was not willing to go on air while journalists had to run enormous risks to cover the news.

Monterrey, the richest city in Mexico and its industrial capital, has seen a sharp increase in crime since one of the country's most violent drug gangs extended its operations there.

On Augusat 14, alleged members of the Zetas cartel blocked off at least 13 major roads in Monterrey.