Gunmen fired on a military helicopter in western Mexico on Friday, killing three troops as the aircraft made an emergency landing in a day of violence across drug crime-plagued Jalisco state.

The attack on the Cougar helicopter came as more than a dozen buses and trucks were torched in Guadalajara, Mexico's second biggest city, and a gang clashed with police in the town of Autlan.

Five banks were set on fire in the town of Ciudad Guzman, authorities said.

Authorities have not said who was responsible for the violence, but it came amid an escalation of violence by the Jalisco New Generation Drug Cartel, which killed 20 police officers in two ambushes in March and April.

The military helicopter was taking part in Operation Jalisco, which had just launched on Friday to combat crime, when it spotted carloads of gunmen in the southwestern part of the state.

"(The gunmen) attacked the military personnel with shots from firearms, hitting the Cougar helicopter, which conducted an emergency landing," the defense ministry said in a statement,

The helicopter was carrying 11 troops, five crew members and two federal police officers. They had spotted the gunmen on a road between the towns of Casimiro Castillo and Villa Purificacion when they were hit.

The two police officers and 10 soldiers were injured. They were taken to the military hospital of Guadalajara.

The assailants left behind four vehicles.

The location of the attack was near the town of Autlan, where a clash was reported between a gang and the authorities.

- Clash, buses torched -

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

Starting at around 9:45 am in Guadalajara, buses and trucks were torched in at least a dozen streets, but Sanchez said no shootouts were reported in the western city.

Authorities did not say who set them on fire or why, but such tactics were used by the Gulf cartel in northern Tamaulipas state following major arrests last month.

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. Vehicles were torched in other parts of the state.

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

- Operation Jalisco -

Before the violence erupted, the government announced the launch of Operation Jalisco, which includes the navy, the army, the federal police and the attorney general's office "to guarantee the security of citizens and reduce crime."

The western state of Jalisco is known as the birthplace of tequila, mariachis and the country's most popular football team, the Chivas of Guadalajara.

But now it is also known as the home of the New Generation, a rising power of Mexico's underworld that had been overshadowed until now by other groups such as the Sinaloa, Zetas, Gulf and Knights Templar cartels.

Officials say the Jalisco cartel has grown so powerful that it has produced its own assault rifles in makeshift gun assembly shops. The gang has even recruited military deserters, including foreign ones.

The cartel has drawn the attention of the U.S. government, which has funded Mexico's battle against drug cartels by providing equipment, training and intelligence.

Last month, the U.S. Treasury Department slapped financial sanctions against the New Generation and its shadowy boss, Nemesio Oseguera, alias "El Mencho," as well as its allies, the Los Cuinis cartel.

The gang has expanded beyond Jalisco into neighboring Michoacan and Colima while forging ties with other criminal organizations in the United States, Latin America, Africa, Europe and Asia, according to the U.S. Treasury.