The headless bodies of 18 people have been found in two abandoned cars in Mexico.

The bodies, which are so badly mutilated forensic teams have been unable to determine their sex, were left by the side of a road in the western state of Jalisco. It is thought the victims were left there by either the Zetas or Sinaloa drug cartels that are battling each other for control of the area.

Police say they found the cars near Lake Chapala, an area popular with tourists and foreign residents, after receiving an anonymous tip-off. Investigators initially thought there were only 12 bodies, but later upped this figure to 18 after picking through the body parts. They added that a threatening note had been found in the cars indicating the Zetas cartel was behind the killings.

The latest gruesome find is one of a series of mass killings in Mexico in recent months.

The deaths come just a few days after the bodies of four people -- two of them journalists - were found in bags and dumped in a canal in Boca del Rio.

They'd apparently written stories challenging the gangs and one of the victims was said to have received death threats weeks earlier.

On the same day another 14 bodies were found dismembered in the border city of Nuevo Laredo, and nine people were discovered hanging from a bridge there.

Last November, the corpses of 26 people were found in three cars in Guadalajara.

In the last five years, since President Felipe Calderon came to power, more than 50,000 people have died in drugs battles in the country.