GUADALAJARA, Mexico (AFP) - Armed men shot at a Mexican army helicopter in the western state of Jalisco on Friday, killing three military personnel and injuring 12 others as the aircraft was forced to make an emergency landing, the Defence Ministry said.

Violence in Jalisco, one of Mexico's most important states economically, has become an increasing problem for President Enrique Pena Nieto, who took office pledging to pacify the country following years of brutal drug gang violence.

The Jalisco New Generation cartel, which is based in the state, was believed to be behind the attack, according to a government official, who spoke on condition of anonymity.

Violence also flared up on Friday in Jalisco's capital Guadalajara, the second-largest city in Mexico, with vehicles set ablaze in and around the metropolitan area.

"This is a reaction to an operation we are doing to get to the bottom of, and detain leaders of this cartel," Jalisco Governor Aristoteles Sandoval said.

Sandoval said that at least seven people had been killed in Jalisco, and another 15 injured.

A total of 12 banks and 16 gas stations were damaged by the violence, he added, as local and social media showed images of buses and buildings set on fire.

The government said three other security personnel are still missing after the attack on the helicopter, which took place by a highway near the Pacific coast of southwestern Jalisco.

Over the past two months, security forces have come under attack in Jalisco from organised crime, with at least 20 federal and state police killed in separate incidents.

With an economy bigger than Kenya's, Jalisco is home to close to eight million people and accounts for roughly 6.5 per cent of Mexican gross domestic product. It is also the heartland of tequila and Mexico's mariachi musicians.

Though the official homicide toll in Jalisco has fallen since Pena Nieto took office in December 2012, the President identified the state as one of the areas most at risk when he set out plans to improve security in Mexico last November.

Since the start of 2007, more than 100,000 people in Mexico have died in violence stemming from turf wars among the gangs as well as their clashes with security forces.

More than a dozen vehicles were set on fire on Friday across Guadalajara, Mexico's second-biggest city.

At least one police officer was killed and "an undetermined number" of gang members died in a shootout in the town of Autlan, south-west of Guadalajara, state government spokesman Gonzalo Sanchez told Milenio television.

In the western city of Guadalajara, buses and trucks were torched in at least a dozen streets.

Authorities did not say who set them on fire but such tactics have been used by drug gangs in other parts of the country following major arrests.

The Jalisco New Generation cartel, the dominant gang in the state, has defied the authorities this year, launching two deadly ambushes in March and April that killed a total of 20 federal and state police.

Authorities urged residents of Guadalajara to stay indoors on Friday.

"Remain calm. If you have any reason to leave your house, don't go out," the Jalisco state prosecutor's office said on Twitter.

Authorities said they had begun to douse the burning vehicles in Guadalajara.

"The entire government is active. We are redoubling security in the state," Governor Aristoteles Sandoval wrote on Twitter.