Mexican police have found six charred bodies on a street in the northern border city of Tijuana, bringing the number killed in three days to 14.

The six men had their hands bound and heads covered with plastic bags and had been shot or beaten, officials said.

The execution-style killings followed a period of relative calm in Tijuana, where fighting between rival drug gangs has left some 300 dead this year.

Some 25,000 Mexican troops have been deployed to tackle the drug cartels.

The six bodies were found on a Tijuana street early on Monday morning and were believed to be the victims of a drug gang attack, police said.

"Some of the victims were shot dead or beaten. It's not clear if any were burned alive," a police spokesman told Reuters news agency.

Tijuana, across the US-Mexico border from San Diego, has been caught up in the three-way fight between rival drug gangs and the security forces.

Mexico's most wanted man Joaquin "Shorty" Guzman is trying to win control of smuggling routes into California from the Arellano Felix cartel that has long held sway in Tijuana.

Unprecedented violence

The city had been relatively quiet until the outbreak of violence over the weekend.

"It's a situation that obviously worries us," the assistant Baja California state attorney Salvador Ortiz told the Associated Press news agency.

The Mexican government has admitted there would be no quick end to the violence and that the army will be used for at least another two years in the fight against the drug gangs.

Mexican and US anti-narcotics officials say higher street prices indicate fewer drugs are getting into the US and that the increased violence shows the cartels are battling for a slice of an ever smaller pie.

However, more sceptical voices say the unprecedented level of killings - at least 1,400 so far this year - and the high-profile murders of Mexican police chiefs show the drugs gangs are far from being controlled.