Mexican authorities have discovered two human heads in a cooler near the resort city of Cabo San Lucas, in the Baja California peninsula. The discovery occurred days after the state prosecutor's office said two dismembered bodies had been found in another cooler in the area, news agency AP said.

Authorities also discovered at least 18 corpses in clandestine mass graves in the area, following days of excavation.

The bodies of 13 men and five women were recovered from 12 graves off a road that runs between the Cabo Pulmo nature reserve and the resort of San Jose del Cabo.

Authorities attributed the recent killings to turf war among drug gangs.

Mexico Daily News said it is believed violence is linked to the Sinaloa, Jalisco Nueva Generación and Tijuana cartels.

The news site also said concern is growing that the recent spate of violence might affect tourism in the area, known for its beaches and night-life.

This is not the first time that dismembered bodies hidden in coolers have been discovered in Mexico.

In 2015, the butchered body of a female assassin thought to be working for Los Ciclones, one of the factions of the Gulf Cartel, was discovered in a beer cooler in Lauro Villar, Matamoros.

Some 30,000 people have disappeared in Mexico since drug war escalated in 2007, according to Reuters.

The disappearance of 43 college students from the city of Iguala, Guerrero state, in 2014 shed light on the extent of violence in the area.

The students, from the Ayotzinapa Teacher Training College, disappeared after staging a protest against what they perceived to be an unfair hiring process for teachers, which favoured urban applicants over rural ones.

The government said the students were abducted and handed over to the Guerrero Unidos drug cartel, who killed them and incinerated their bodies. However, some parents and independent experts have contested these claims.

In December 2014, an Austrian forensics lab identified the body of one of the students, whose body parts were found at a rubbish dump near Iguala.

Some 80 people, of whom 44 were police officers, have been arrested in connection with the students' disappearance.

The students' disappearance sparked violent rallies across Mexico, with people protesting against perceived corruption and calling for President Pena Nieto to step down.

The drug war in Mexico was ranked 2016's second deadliest conflict in the world, with only the war in Syria being labelled worse, a new annual survey has revealed. This means Mexico experienced levels of violence far more extreme than that of Iraq, Yemen or Afghanistan.