A survey released this week showed that half of foreign firms operating in Mexico were considering scaling back their presence because of the rising drug violence.

The survey by healthcare firm International SOS said 33.6 per cent of foreign executives in Mexico had already reduced their operations, while more than a third of those surveyed said that their organisations had experienced "criminal activity or attempted criminal activity" in Mexico in the last 12 months.

Risk

"The key finding in the survey is that companies that do business in Mexico have found it ... to be a little bit more challenging because of the growing violence in Mexico," Alex Puig, International SOS' director of security for the Americas, told Reuters news agency.

More than 26,000 people have died since Mexico's drug war began in 2006 [Reuters]

"The real finding within that is that they are actually paying a lot more attention than they were before to managing the risk of their travelers into Mexico," he added.

The city of Monterrey has been a key focus of foreign investment in Mexico, and has long considered itself separate from the crime and corruption rife in other parts of the country.

But the recent surge in violence in the city between the between the powerful Gulf cartel and its brutal former armed wing, the Zetas, has forced many companies to freeze investments amid worries over the safety of doing business there.

A day before the latest bodies were found Mexican soldiers uncovered five communal graves together with three large containers with human remains inside in the municipality of Guadalupe, within Monterrey, following an anonymous phone tip-off.

The bodies in the containers had been partly diluted in acid.

On Monday the bodies of five men, all showing signs of torture, were found in an abandoned van in a Monterrey side street.

Two of the victims were reportedly policemen kidnapped two days earlier and one had been beheaded.

The deaths this week bring the number of people killed in Monterrey since the start of the year to almost 300.

Intimidation

At the same time, there is growing evidence that growing numbers of Mexican police officers are quitting their jobs because of intimidation from drug gangs.

On Wednesday eight police resigned from a local force in the town of Navolato in the western state of Sinaloa.

Navolato is considered one of Mexico's most dangerous towns and some 70 police have quit the force their since January, while 16 have been killed in action.

Across Mexico more than 26,000 people – mostly drug traffickers and police – have been killed since December 2006 when Felipe Calderon took office as president launching his war on the country's drug gangs.