Hundreds of thousands of people in the US state of West Virginia have been left without water after a chemical leak into a key river, officials said Friday.

West Virginia American Water said it had ordered residents across a wide region not to use water after chemicals from a plant were pumped into the Elk River.

"Do not use the water," the firm said, in an advisory to customers. "Due to the nature of the contamination, the water should not be used for any purpose other than toilet flushing and firefighting."

According to the Charleston Daily Mail local newspaper, some 300,000 people were affected by the restrictions.

Governor Earl Ray Tomblin said authorities were working to restore the water supply after the contamination.

"Our efforts will continue until we have a resolution," he said. "Our main focus continues to center around our hospitals, nursing homes and those most vulnerable."

The substance inadvertently pumped into the river -- methylcyclohexane -- is commonly used to clear coal of impurities.

West Virginia American Water chief Jeff McIntyre told the Charleston Daily Mail that the chemical may have contaminated up to 1,500 miles (2,400 kilometers) of pipelines.

He said officials were still working to assess the exact nature of the threat the chemical may pose to humans.

The chemical was not thought to have a "high lethality" but officials would not take any chances, he said.

"We haven't had a situation like this where we've had a 'do-not-use' (advisory) of this magnitude," McIntyre said.