At least 16 people have been killed in a series of gang-related shootings in the Mexican Gulf coast state of Veracruz.

A spokeswoman for the state government told local television on Thursday, that gunmen killed four people in the small town of El Higo then attacked three buses on a highway about 100km southwest of the port of Tampico, killing seven others.

Five armed men were then killed in a shootout with security forces, she said.

The death toll from the bus shootings could be as high as 20, a local mayor told Milenio television.

The motive was unclear. The attacks came the day after the state government said it disbanded the municipal police force in the city of Veracruz, some 480km south of Tampico.

As part of efforts in Mexico to root out corruption in local law enforcement, roughly 1,000 police officials are temporarily being replaced by the navy in Veracruz and the neighboring municipality of Boca del Rio, local media said.

The state is home to several of Mexico's major oil installations and the important oil-exporting port
Coatzacoalcos, just south of the city of Veracruz.

Between late September and early October, close to 100 bodies were found dumped in Veracruz city during an outbreak of drug violence, prompting the federal government to send in reinforcements.

Officials blamed the bloodletting on a group linked to the country's most powerful drug lord, Joaquin "Shorty" Guzman.

He has been fighting a turf war with the violent Zetas cartel, which authorities say controls Veracruz.

More than 45,000 people have been killed since President Felipe Calderon launched a war on drug cartels in late 2006.

In August, Calderon said drug gangs, which can offer police officers much higher pay than the state, had corrupted all levels of government in Mexico.