The US environmental regulator on Thursday asked gas companies to reveal what chemicals are used in deep extraction, addressing concerns by residents that their drinking water is being contaminated.

US companies have increasingly tapped shale gas, which lies deep underground and was once thought inaccessible. Firms force out the gas through hydraulic fracturing, or fracking, in which a large volume of liquid is injected below.

The Environmental Protection Agency said it had sent letters to nine companies including energy giant Haliburton asking for data within 30 days on the chemicals involved in fracking.

The agency's chief, Lisa Jackson, said the data would "help us make a thorough and efficient review of hydraulic fracturing and determine the best path forward."

"Natural gas is an important part of our nation's energy future, and it's critical that the extraction of this valuable natural resource does not come at the expense of safe water and healthy communities," Jackson said.

The agency said it was asking the companies to turn over data voluntarily but could try to force them if they did not comply.

Under a law signed by then president George W. Bush in 2005, fluids used in fracking were exempted from environmental laws such as the Clean Water Act.

But a number of communities have since complained that fracking has polluted their drinking water.

In the documentary "Gasland," which won the Special Jury Prize at this year's Sundance Film Festival, filmmaker Josh Fox showed families in a drilling area whose tap water was even flammable.

President Barack Obama's administration last month held a first-of-a-kind international conference to promote shale gas, calling it a plentiful resource that can curb emissions of carbon blamed for climate change.