Federal government oversight of hydraulic fracturing – a drilling technique that boosts natural gas extraction by blasting water, sand and chemicals underground at high pressure – is sorely lacking, putting drinking water supplies at risk, an environmental policy group claimed in a report released Tuesday.

According to the Environmental Working Group, a research and advocacy group based in Washington, drilling companies are side-stepping a permitting requirement for the use of diesel fuel in their fracturing fluids by using similar petroleum distillates that contain the same toxins as diesel, but require no permitting.

The report also cites evidence that drilling companies continue to inject diesel fuel underground without the proper permits.

“The industry has been able to operate above the law,” said Dusty Horwitt, the group’s senior counsel. “They’re doing an end-run around what little oversight is left.”

The use of hydraulic fracturing has vastly boosted the productivity of existing natural gas wells and opened new territory to drilling, helping lift natural gas reserves in the United States by nearly a third, and igniting a drilling boom from New York to Wyoming.

Yet as gas companies explore drilling in sensitive regions like the New York City watershed in upstate New York, concern over the technology’s potential to pollute water supplies has mounted.

The drilling boom – and the growing controversy over the risk of water contamination – was outlined in a December article by Times reporters Jad Mouawad and Clifford Krauss.

As they noted, the evidence of groundwater pollution attributable to fracturing is thin — though environmental groups contend that is because governments have been slow to react to the drilling boom and are not looking hard for contamination.

Gas companies, meanwhile, acknowledge the validity of some concerns, but they claim that their technology is fundamentally safe.

In 2004, the United States Environmental Protection Agency released a report declaring that hydraulic fracturing posed little threat to human health. The following year, drawing support from the E.P.A. report, Congress voted to exempt hydraulic fracturing from oversight under the Clean Water Act.

The law made one exception, however – that companies acquire permits when using diesel fuel in their fracturing fluid. Diesel contains benzene and other cancer-causing agents.

Obeying the letter of the law, many drilling companies switched from diesel to other petroleum distillates. Yet these distillates also contain toxics such as benzene, sometimes in even greater levels than the diesel they are replacing, according to the Environmental Working Group report.

“These substitutes are extremely toxic,” Mr. Horwitt said.

The group also contacted environmental agencies in New York, Pennsylvania, Montana, Texas and Wyoming – states where hydraulic fracturing has been used extensively – seeking information on the continuing use of diesel fuel in drilling.

Only one state, Wyoming, provided information on permitting, the report states.

“It’s not even clear if companies are getting permits if they’re using diesel,” Mr. Horwitt said. “This whole area has been shrouded in secrecy and lax regulation.”

Drilling companies maintain that hydraulic fracturing is safe and have launched a fierce lobbying and public relations effort to resist further regulation.

Congress will have a chance to revisit the controversy over the safety of the technology this week as it weighs approval of the Exxon Mobil Corporation’s proposed $41 billion acquisition of XTO Energy, a major player in natural gas extraction.