At least six people have been killed in a grenade attack at a casino in northern Mexico.

State officials said gunmen Thursday threw at least one grenade into the casino in the city of Monterrey. The blast sparked fires and witnesses reported dense smoke around the building.

Some local media are reporting a higher death toll of at least eight people killed. At least five others were injured in the attack.

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.