Morelia (Mexico) (AFP) - A police helicopter crashed during an operation to capture suspected gang leaders in western Mexico, and authorities were investigating whether gunmen shot it down, killing three officers and the pilot.

Silvano Aureoles, governor of the troubled western state of Michoacan, wrote on Twitter that the helicopter "was downed" during the operation in an area of rough terrain.

But hours later, he told the Televisa network that he "can't confirm that it was downed" and that authorities were investigating the cause of the crash.

Officials are hoping to speak with an officer who survived the crash but was in intensive care.

The incident took place in Michoacan's Tierra Caliente, a region that has endured years of drug violence and vigilante justice.

If confirmed, it would be the second time that a gang downs a helicopter since 2015. Last year, the Jalisco New Generation drug cartel in neighboring Jalisco state used a rocket launcher to hit a military helicopter, killing seven soldiers and policewoman aboard.

In a press conference, Aureoles said the helicopter fell during an operation to capture leaders of an unidentified criminal group who had tried to abduct a farmer in La Huacana.

"Be certain that we will free Michoacan of the scourge of impunity and crime," Aureoles said.

Aureoles said authorities have conducted operations to capture gang leaders in Tierra Caliente and Sierra Costa that, since August 30, led to a dozen arrests and the seizure of several weapons, including two rocket launchers.

The pseudo-religious Knights Templar drug cartel terrorized Tierra Caliente until lime growers formed vigilante forces in 2013 to fight back against the gang.

The cartel was weakened as authorities arrested or killed its top leaders, but smaller criminal groups have since emerged.

The vigilantes have been ordered to disband but some of the militia's members have been implicated in crimes.

Homicides are on the rise in the state, with 678 murders in the first seven months of the year compared to 777 in 2015, according to federal government figures. At least 150 people were killed in July alone, a twofold increase from the same month last year.