By Rory Carroll

SAN FRANCISCO (Reuters) - An environmental group threatened on Thursday to sue the federal government unless it halts offshore fracking in California immediately, the latest legal challenge levied against the controversial oil extraction technique.

The Center for Biological Diversity filed a notice of intent to sue the U.S. Interior Department, saying it violated three federal laws by allowing fracking in California’s Santa Barbara Channel without evaluating its polluting effects on coastal communities or marine wildlife such as blue whales and otters.

Oil companies have fracked at least 21 times in federal waters off the California coast with no analysis of environmental risks and no determination of whether it violates a state coastal management program, the group said.

Several offshore fracking jobs took place in an area that is part of a proposed national marine sanctuary, the Center said.

If the federal government does not halt the practice to evaluate its safety within 60 days, the Center says it will sue.

Hydraulic fracturing, or fracking, involves injecting water and chemicals deep underground to break up rock and release oil and gas.

Fracking, in combination with other oil drilling techniques, has helped lead to a domestic energy boom, although its impact on California oil production has been less dramatic than in other regions. The practice has also led to concerns about its impact on health and the environment.

“The federal government’s turning a blind eye as offshore fracking threatens to poison our beautiful beaches and coastal waters,” said Miyoko Sakashita, an attorney and director of the group’s oceans program.

“We need offshore fracking stopped immediately before chemical contamination or an oil spill devastates California’s coastal communities and kills sea otters and other endangered marine wildlife,” she said.

A spokesman for the Western States Petroleum Association, an oil industry trade group, declined to comment on the potential lawsuit.

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On Wednesday, the Environmental Defense Center filed a lawsuit against the Bureau of Safety and Environmental Enforcement, saying it failed to provide for public or environmental review prior to approving 51 oil drilling permits that allow for fracking in the Santa Barbara Channel.

“We call on the Obama administration and Interior Department to put a halt to offshore acidizing and fracking unless and until they can be proven safe for California’s irreplaceable coastline,” said Brian Segee, an attorney with EDC.

(Reporting by Rory Carroll)