China's state-run news agency say police have arrested more than 12,000 people and seized 300 kilograms of illicit drugs in a nationwide crackdown on narcotics sold through online chat rooms.

The official Xinhua news agency says authorities were alerted to the online sales after uncovering chat rooms in the western cities of Lanzhou and Xi'an that were selling drugs.

Xinhua says newcomers could only join the chat rooms if they were introduced by existing users.

Police began arresting suspects in September.