Six severed heads have been found on a road in central Mexico, in an area not normally associated with cartel violence.

Local authorities made the discovery early on Tuesday morning on a route that links the broadly peaceful states of Puebla and Tlaxcala.

Police have not given a motive for the killings or said which of the criminal groups operating in Mexico might have carried them out.

Local media has reported that a blanket was left at the scene with a message issuing a warning to rival gangs and apparently signed by a group called "La Barredora", meaning the sweeper.

It is the same name as a little-known criminal group operating in the western state of Guerrero but it isn't clear if they were behind the attack or why.

The local prosecutor's office said the heads found in Tlaxcala were those of men and it has launched an investigation into the killings, according to AFP news agency.

As well as drug-trafficking, there is an issue in the region with fuel smuggling, known as "huachicolea", which generates billions of dollars a year for the groups behind the illegal activity.

So far, federal authorities have not commented on the killings.

They come amid a major crackdown by the administration of President Claudia Sheinbaum on fentanyl trafficking.

Puebla and Tlaxcala are not prone to the kind of extreme cartel violence prominent in other parts on the country.

In June, the bodies of 20 people - four of them decapitated - were found in Sinaloa, a state gripped by gang violence.

Seven Mexican youths were also killed in a shooting at a Catholic Church festivity in the central state of Guanajuato in May.

Violence between cartels has surged in recent years, with hundreds of thousands of people killed and tens of thousands missing since the government first began to use the Mexican military against gangs in 2006.