China has punished 20 officials in the northwestern province of Gansu after more than 2,000 people, including some 300 children, suffered lead poisoning from waste discharged by a smelter, state media reported on Tuesday.

Investigators determined in 2006 that the local environmental protection bureau had violated laws in approving construction of the plant, now known to be the source of the contamination.

Ran Hong, chairman of the Huixian non-ferrous metal smelter company, would be charged with violating environmental laws, and the other 19 officials were either sacked or received warnings, the Xinhua news agency said.

The report did not name the other officials or provide details of their punishments.

"The company had violated regulations, discharging waste exceeding the environmental standards, and did not use anti-pollution equipment, which was the main reason of the incident," the news agency said, citing the Gansu government.

Officials from the local government and the environmental authorities held "unshakable responsibilities", it added.

Beijing has been trying to tighten monitoring of potential environmental hazards and curb pollution, but it has acknowledged it is often stymied by local officials who allow or overlook violations because they are accustomed to being judged on economic growth at any cost.