At least 20 people have been killed after armed assailants set fire to a casino in northern Mexico.

Monterrey Mayor Fernando Larrazabal told reporters Thursday people remain trapped inside the burning building and rescue efforts are ongoing.

Initial accounts reported the gunmen had used grenades, but state officials now say the attackers doused the casino in a flammable liquid and set it ablaze.

Witnesses reported dense smoke around the building.

The motive for the attack is so far unknown. However, drug-related violence is on the rise in Monterrey, on the border with the United States.