Washington (AFP) - Hydraulic fracturing to release natural gas and oil can have a negative impact on the quality and availability of drinking water in the United States, the US government said Tuesday.

"This assessment is the most complete compilation to date of national scientific data on the relationship of drinking water resources and hydraulic fracturing," said Thomas Burke, the Environmental Protection Agency's science adviser.

The final EPA report "provides scientific evidence that hydraulic fracturing activities can impact drinking water resources in the United States under some circumstances," and is based on a review of over 1,200 cited scientific sources.

Hydraulic fracturing, or fracking, involves pumping large quantities of water and chemicals at high pressure deep into the Earth to fracture rock to stimulate the flow of natural gas or oil.

The EPA report was issued at the request of Congress.

The EPA's initial assessment had found no evidence that fracking could systematically harm water supplies.

But the final report reversed course, and deleted that sentence.

It also "identified certain conditions under which impacts from hydraulic fracturing activities can be more frequent or severe," including fracking in areas where water availability is low, and disposing of wastewater in unlined pits.

The report did not document specific instances of drinking water impacts.

Nor did it estimate how common fracking pollution of drinking water might be.

"The value of high-quality science has never been more important in helping to guide decisions around our nation's fragile water resources," said Burke.

"EPA's assessment provides the scientific foundation for local decision makers, industry, and communities that are looking to protect public health and drinking water resources and make more informed decisions about hydraulic fracturing activities."