Gunmen stormed a rural town in southern Mexico and killed 13 people, while the U.S. government warned Americans against traveling to cities in a northern border state where shootouts have left slain 19 people over three days.

Hooded assailants in several cars killed nine police officers at a checkpoint in San Vicente Camalote, a town in southern Oaxaca state, state deputy attorney general Netolin Chavez said Wednesday.

The gunmen then burst into the ranch of Alfonso Maciel, killing him and his three sons, one of whom was a minor, Chavez said. He said investigators believe the attack Tuesday night was related to drugs, but he had no other details.

State police were sent to patrol the town.

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.

The U.S. Consulate in Monterrey on Wednesday warned American citizens to avoid traveling to Nuevo Laredo, across the border from Laredo, Texas, and Reynosa, which faces McAllen.

Mexican security forces have fought gang members in several cities of Tamaulipas state since Sunday, said Hernan de la Garza, the state deputy attorney general.

Among the 19 dead were a soldier and a police officer, both killed in attacks in two different towns Tuesday.

Gunmen also attacked the police headquarters in the town of Miguel Aleman on Tuesday, de la Garza said. Six officers are missing and presumed kidnapped.

As rumors of shootouts spread, people stayed off the streets and some two dozen schools ended classes early Tuesday in Nuevo Laredo and Matamoros, which lies across from Brownsville, Texas. Reports of fighting spread by e-mail, Twitter and other social network postings.

There were no shootings in either city 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.

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

The massacre in Oaxaca was the second mass killing in Mexico in less than a month.

On January 30, gunmen killed 15 people in a working-class neighborhood of Ciudad Juarez, a city facing El Paso, Texas, that has become one of the world's deadliest places. Most of the dead were teenagers at a party with no known ties to drug gangs.

Authorities say the gunmen may have been acting on mistaken information _ fueling anger over a drug war that has increasingly taken a toll on civilians.

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