By Karen Brooks

(Reuters) - West Virginia officials said Thursday they have lifted a ban on drinking tap water for two-thirds of the customers affected by a chemical spill, but warned pregnant women to avoid it until the chemical is completely flushed from the pipes.

One week after the spill into the Elk River prompted authorities to order some 300,000 people not to drink or wash with their tap water, officials have cleared more than 200,000 of them to start drinking the water again after tests showed levels below the 1 part per million level safety standard set by the federal Centers for Disease Control and Prevention.

But pregnant women should continue to steer clear of the water in an "abundance of caution" until the chemical is completely undetectable, West Virginia American Water said.

The company said the CDC had advised there is still a "limited availability of data" on whether pregnant woman are more susceptible and advised that state water officials "consider an alternative drinking water source for pregnant women until the chemical is at non-detectable levels in the water distribution system."

The state attorney general, the U.S. Chemical Safety Board and the U.S. Attorney for the Southern District of West Virginia are investigating the January 9 leak of about 7,500 gallons (28,000 liters) of 4-methylcyclohexane methanol, or crude MCHM, into the river.

Officials ordered water use halted for everything but flushing toilets after the leak from a storage tank owned by Freedom Industries, a maker of specialty chemicals.

The Freedom Industries site has not been inspected since 1991 and is about a mile upstream from a West Virginia American Water plant, the biggest in the state. Crude MCHM is used in washing coal and Freedom Industries has apologized for the incident.

Downstream from the spill, the Northern Kentucky Water District and the Greater Cincinnati Water Works have shut their intakes on the Ohio River as a precaution.

Water tainted by crude MCHM smells faintly of licorice. Contact with the water can cause nausea, vomiting, dizziness, diarrhea, rashes and reddened skin.

West Virginia American Water is a unit of American Water Works Co Inc.

(Reporting by Karen Brooks; Editing by Scott Malone and Sofina Mirza-Reid)