West Virginia on Monday began lifting a ban on tap water imposed in the state last week after a chemical leak into a key river.

Hundreds of thousands of people in the Appalachian state had been without water except for flushing toilets since Friday when officials disclosed that chemicals from a plant had been pumped into the Elk River.

Water company officials on Monday said testing had shown that water quality was now at levels deemed safe, allowing the ban to be lifted in stages.

"The first zone, which includes the downtown Charleston and East End areas, has now been lifted," West Virginian American Water said.

"The ban is being lifted in a strict, methodical manner to help ensure the water system is not overwhelmed by excessive demand, thereby causing more water quality and service issues," it said.

Customers, however, were instructed to first flush their home plumbing systems before using the water once the ban had been lifted in their area.

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.

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