Later three more officers were killed in a shootout with gunmen in the drug-plagued northern border city of Ciudad Juarez.

"If we remain with our arms crossed, we will remain in the hands of organised crime, we will always live in fear"

Felipe Calderon,
Mexican president

The latest in a series of mass slayings came as Felipe Calderon, the Mexican president, defended his crackdown on drug gangs in an essay on his office's website.

Calderon vowed no let up in the campaign despite criticism that violence has only surged since he deployed thousands of troops and federal police in late 2006 seeking to crush the drug gangs.

"I'm clearly convinced that we would be in a much worse situation if we hadn't decided to fight criminals," he wrote in the 5,000-word essay, which was published by several newspapers.

"If we remain with our arms crossed, we will remain in the hands of organised crime, we will always live in fear."

Tortured

Calderon also blamed Mexico's violence on the high demand for illegal drugs from across the border in the United States.

Last year in an attack on law enforcement the bloodied and tortured bodies of a dozen federal agents were found dumped along a highway in Michoacan state, the place where Calderon first launched his crackdown.

The US State Department has issued a travel warning urging US citizens "to exercise extreme caution" when travelling in Michoacan.

The state is known as a stronghold of the La Familia cartel, known for beheading its rivals and making bold attacks on government security forces.

At least 23,000 people, mainly traffickers and police, have been killed in drug-related violence since Calderon launched an army-led offensive against drug gangs after taking office in December 2006.