The U.S. government warned its citizens on Wednesday against travelling to a northern Mexico border state where shootouts killed 19 people the previous three days. The alert came a day after assailants stormed a rural town in southern Mexico and killed 13 people.

President Felipe Calderon, addressed the upsurge in violence, using a rare news conference with local news media to hotly deny accusations that his government is favouring one drug cartel over another.

Mr. Calderon called the accusations “incredible ... false and ill—intentioned, and I don’t know what motives lie behind them.”

In Nuevo Laredo, across the border from Laredo, Texas, reports of violence and impending violence have grown so bad that the U.S. Consulate in Monterrey warned American citizens on Wednesday to avoid travelling there.

Nineteen people, including a soldier and police officer, were killed over three days in shootouts across the state of Tamaulipas, where Nuevo Laredo is located. Security forces have fought gang members in several areas of Tamaulipas since Sunday, state deputy attorney general Hernan de la Garza, said.

On Tuesday, gunmen attacked the police headquarters in the town of Miguel Aleman, de la Garza said. Six officers were missing and presumed to have been kidnapped.

As rumours of shootouts spread, people stayed off the streets and two dozen schools ended classes early Tuesday in Nuevo Laredo and Matamoros, which sits across the border from Brownsville, Texas. Reports of fighting spread by e—mail and social network postings.

There were no shootings in either city on Tuesday, however. Nuevo Laredo school superintendent Aurelio Uvalle, said schools did not have permission to close and teachers who failed to show up for classes could face disciplinary measures.

Drug battles had quieted somewhat in Tamaulipas the last few years, but the state has seen a recent increase in clashes. On Wednesday, the navy said three of its marines died in a shootout that also killed three suspected gang members on February 8 in the border town of Reynosa. It was unclear why the navy waited to release the information.

In the southern state of Oaxaca on Tuesday, hooded assailants kidnapped several police officers at a checkpoint in San Vicente Camalote, state officials said on Wednesday.

The gunmen then burst into a house where they killed rancher Alfonso Maciel and five other people, including three police officers, before moving on to Maciel’s ranch, where they killed four people, including three of his sons. One of the sons was a minor.

As they fled, the gunmen killed three local policemen who tried to stop them. One policeman was reported missing and possibly kidnapped by the gunmen, and state police were sent to patrol the town. Maciel apparently had links to a drug cartel, the state government said in a statement.

The government said the 30 to 40 attackers travelled in five or six vehicles with license plates from the neighbouring state of Veracruz - territory of the Gulf cartel.

Oaxaca, a Pacific coast state with a picturesque colonial capital, ancient Zapotec ruins and pristine beaches, is considered a drug trafficking corridor, although violence has not risen to the levels plaguing Mexico’s northern border region.

In Guerrero, another Pacific coast state considered a trafficking corridor, police found two severed arms in an icebox on Wednesday, along with a threatening message. State police said an anonymous tip led them to the remains in the town of Ciudad Altamirano.

The Gulf cartel has recently hung banners in Nuevo Laredo accusing Calderon’s administration of protecting the less—openly bloody Sinaloa cartel, while cracking down on extremely violent rival drug gangs.

While Sinaloa hit men have carried out massacres in the past, the Gulf and La Familia cartels frequently use the grizzliest methods imaginable to eliminate rivals and attack police and army patrols.

Mr. Calderon angrily denied the accusation of favouritism on Wednesday.

“This administration has attacked all the (drug) groups, without distinction,” he said.

In a separate statement, the Mexican Interior Department said that of the almost 72,000 people detained on drug charges during Mr. Calderon’s administration, 27 percent belonged to the Gulf cartel, just slightly more than the 24 percent from the “Pacific” (Sinaloa) cartel.

But the Gulf has borne the brunt in terms of arrests of top leadership.

Osiel Cardenas, the chief of the Gulf cartel who was arrested in Mexico in 2003 and extradited to the United States in 2007, was sentenced to 25 years in prison on Wednesday and ordered to forfeit $50 million by a court in Houston.

Some 15,000 people have died in drug gang violence since President Felipe Calderon took office in 2006 and deployed thousands of troops and federal police to root out Mexico’s brutal cartels.

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