Six underground tanks that hold a brew of radioactive and toxic waste at the most contaminated US nuclear site are leaking.

The leaks raise new concerns about delays over emptying the tanks at Washington state's Hanford nuclear site.

And they strike another blow to efforts to clean up the site, where successes are often overshadowed by delays, budget overruns and technological challenges.

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State officials just last week announced one of Hanford's 177 underground tanks was leaking between 150 gallons to 300 gallons a year, posing a risk to groundwater and rivers.

Governor Jay Inslee said yesterday the leaking material poses no immediate risk to public safety or the environment because it would take a while - perhaps years - to reach groundwater.

He travelled to Washington DC this week to discuss the problem with government officials.

And he said he received the "very disturbing news" during meetings yesterday that six tanks are leaking.

"I think that we are going to have a course of new action and that will be vigorously pursued in the next several weeks," he said.

His comments came days after Democrat Senator Ron Wyden said the Hanford nuclear site - and the challenges associated with ridding it of its toxic legacy - would be the subject of upcoming hearings and a higher priority in the capital.

The federal government created Hanford in the 1940s as part of the top-secret project to build the atomic bomb, and the site went on to produce plutonium for the nation's nuclear weapons arsenal for years.

Today, it is the country's most contaminated nuclear site, with clean-up expected to last decades and cost billions of dollars.

The tanks hold some 53 million gallons of highly radioactive waste - enough to fill dozens of Olympic-size swimming pools - and many of them are known to have leaked in the past.

An estimated 1 million gallons of radioactive liquid already leaked, and the tanks are long past their intended 20-year life span.

AP