The controversial oil and gas extraction technique known as fracking poses no widespread danger to US drinking water, the US environmental agency said Thursday in a long-awaited assessment.

Some water resources have been harmed by the practice formally called hydraulic fracturing, it said, but concluded that the risk is small compared to the number of fracking wells in use.

The evaluation from the Environmental Protection Agency delivered a broad overview of the dangers that fracking -- blasting water, sand and chemicals underground to free up and pump oil or gas -- poses to drinking water in America.

Fracking has led to a boom in the US oil and gas industry, and also created many jobs in states like North Dakota.

But it has been much maligned by environmentalists who say it poses an undue threat to nature.

The study requested by congress found that while fracking has not led to "widespread, systemic impacts on drinking water resources," there are potential dangers to some people's drinking water.

Those vulnerabilities included wastewater and "hydraulic fluids" discharged into drinking water, the report said.

While those risks are worrisome, "they were small compared to the large number of hydraulically fractured wells across the country," the EPA said.

- Low level threat -

In a conference call with reporters, Deputy Assistant EPA administrator Thomas Burke described the threat as "relatively low."

But he emphasized that the study was not a policy recommendation on fracking in the country.

"This study follows the water," Burke said. "The assessment is about improving our understanding of the process."

The study included an analysis of some 950 papers and reports on fracking and water supplies as well as the EPA's own research on the topic.

Burke said he hopes the report will give states, tribes and communities information about how to protect water resources while fracking is conducted.

Supporters of the technique were quick to champion the cause after the EPA's report was released.

"Today's study confirms what we already know. Hydraulic fracturing, when done to industry-standards, does not impact drinking water," said US Senator Lisa Murkowski, a Republican who chairs the Energy and Natural Resources committee.

Republican Lamar Smith of Texas, who chairs the House of Representatives Committee on Science, Space, and Technology, also said the agency's finding Committee came as no surprise.

"As experts have testified before the Science Committee for years, there is no evidence that hydraulic fracturing has caused ground water contamination," he said in a statement.

"The EPA has spent millions of taxpayers' dollars only to conclude what state regulators have known for years," Smith said.

"After years of demonization from opponents, the administration has confirmed what we in Texas have known all along: fracking is a safe and reliable way to take advantage of our vast domestic energy resources," said another Texas Republican, Senator John Cornyn.