Authorities work at a crime scene where gunmen killed several people after storming a water park in Cortazar, Mexico, on Saturday. Reuters

Footage widely shared on social media showed the aftermath of the attack in a palm-studded resort in the small town of Cortazar, about 65km south of the Guanajuato city.

It was not clear who was behind the shooting that killed the seven-year-old, three men and three women, Cortazar's local security department said. One person was seriously injured in the La Palma resort.

But in recent years rival drug cartels have been waging brutal battles to control territory and trafficking routes through the state.

Authorities work at a crime scene where gunmen killed several people. Reuters

Video taken soon after the attack showed shocked adults and children walking past piles of dead bodies near a swimming pool.

"Heavily armed sicarios arrived and this is what happened," said an unidentified man, using a word for hired assassins as he filmed at the resort in a video shared on the internet.

Reuters could not independently verify the contents of the video.

"After the attack, (the attackers) fled, but not before causing damage to the resort store and taking the security cameras and the monitor," Cortazar's security department said in a statement.

Reuters