Police have found five severed heads stuffed in a sack outside a primary school in the Pacific coast resort of Acapulco, in Mexico.

The macabre discovery comes after drugs gangs threatened to attack elementary school teachers if they did not pay half their wages to the drugs cartels.

The extortion demands forced around 130 schools in the city to close earlier this month, after administrators and parents decided it was not safe enough to start classes.

The severed heads were found on Tuesday inside a sack that had been placed inside a small wooden crate, the Guerrero state public security secretariat said.

According to Mexican police say the five heads were positioned next to a handwritten message threatening three alleged drug traffickers and Guerrero State Governor Angel Aguirre, who had promised a series of measures to combat the criminal gangs.

A police officer in Acapulco stands next to the sack containing five severed heads which has been placed in a small wooden crate

The message, in an apparently sarcastic tone, told people to thank the governor for continuing “this war”.

All heads appear to be of men, but some of the five headless bodies found elsewhere in the city the previous day were too badly burned to immediately determine their gender.

The horrific event comes just days after police found a woman’s decapitated body in the Mexican border city of Nuevo Laredo, alongside a handwritten sign saying she was killed in retaliation for her postings on a social networking site.

At the end of August, as the new school year began, dozens of teachers in Acapulco said drugs gangs had threatened them with violence if they did not hand over half their salaries from October 1.

Administrators and other personnel also refused to go to work and many schools were left empty and padlocked for two weeks.

The teachers have since been on strike, leading to the closure of more than 100 schools. Earlier this month, they took to the streets to protest at the situation.

Guerrero State Governor Aguirre has promised a series of measures, including increased police patrols and the installation of security cameras and panic buttons in schools.

Even with those security measures, teachers say they still fear for their own and pupils’ safety.