Hitmen from Mexico's drug gangs are breaking traditional codes of honour by killing children in a chilling new chapter of a narcotics war that President Felipe Calderon is struggling to control.

In unprecedented attacks, gunmen killed a 3-year-old boy and a 9-year-old girl and seriously wounded a 12-year-old girl in the city of Tijuana on the U.S. border this week as they targeted a senior local police officer.

Even hardened residents of Tijuana, where more than 300 people were killed in drug violence last year and severed heads were dumped on city streets, were shocked by photos of young Jose Luis Ortiz's body riddled with bullets.

"How much longer must we wait for results from the military? Now the narcos are killing our children," said a Tijuana shop assistant who gave her name only as Fernanda.

Ortiz and his mother and father were shot dead as they slept on Monday night. Gunmen apparently mistook the boy's father for a police officer and had no qualms about killing the 3-year-old.

Moments later, they found the police officer they were looking for and murdered him, his wife and their youngest daughter. Their other child was wounded.

"This is a new strategy to attack children and families and respond to the government's military assault on the cartels. The gangs want to sow panic and fear to overwhelm the authorities," said Victor Clark, a drug trade expert at San Diego State University.

Over the past three decades, Mexican drug cartels hauling cocaine north to the United States have generally held to a code of honour that bans killing women and children and stops them from becoming addicted to the drugs they traffic.

But as the cartels feud over smuggling routes and fight troops and federal police trying to crush them, violence has escalated and many traffickers are now addicts.

Mexican folk singers who praised the escapades of drug smugglers are also being murdered in the conflict.

"We know this is a war and we have to win it every day," Baja California's state governor, Jose Guadalupe Osuna, said after the Tijuana killings.

MORE TROOPS, FEW RESULTS

The main struggle is between the Gulf Cartel on Mexico's eastern coast and an alliance headed by Joaquin "Shorty" Guzman, a jail escapee and Mexico's most wanted man.

Calderon has made crushing the drug cartels a priority since taking office a year ago, sending 25,000 soldiers and police to attack the gangs.

A main focus of the campaign has been Baja California, Mexico's deadliest state last year with more than 400 drug-related murders.

In January, hundreds of police and soldiers rolled into Tijuana and the nearby town of Rosarito to reinforce an already large troop contingent, but street shootouts and daylight kidnappings by hooded men continue.

Baja California officials and Attorney General Eduardo Medina Mora this week promised a redoubling of efforts to stop the violence, but some officials are pessimistic.

"The corruption among the state's police forces runs so deep that it is impeding our work," Gen. Sergio Aponte, joint head of military operations in Baja California, told Reuters. "There are many police officers who have dedicated themselves to protecting criminal interests."