At least six underground tanks containing nuclear waste in the north-western US state of Washington are leaking, but a spokeswoman says there is no imminent threat to public health.

The US Energy Department told the state last week that one tank was leaking at the Hanford nuclear site.

But energy secretary Steven Chu informed its governor Jay Inslee on Friday that more leaks had been discovered.

"Secretary Chu let him know today that there are actually more tanks they've discovered leaking, at least six, possibly more," Mr Inslee's spokeswoman Jaime Smith told AFP.

"At this point we don't believe that there's any imminent threat to public health. Of course we're concerned, because we don't have any information yet about the extent of the leak or how long they've been going on."

Asked for details of the leaking material, she said: "It's nuclear waste. Different tanks have slightly different kinds of waste that they're holding. We're not clear yet on exactly what has been leaking for how long."

The Hanford nuclear site in the south-west of the US state was used to produce plutonium for the bomb that brought an end to World War II.

Output grew after 1945 to meet the challenges of the Cold War, but the last reactor closed down in 1987. Its website says: "Weapons production processes left solid and liquid wastes that posed a risk to the local environment."

It is the Western hemisphere's most contaminated nuclear site, with 200 million litres of radioactive waste stored in aging tanks and billions of dollars a year invested in clean-up.

The ecological threat extends to the Columbia River, it added, noting that in 1989 US federal and Washington state authorities agreed on a deal to clean up the Hanford Site.

The Washington governor's spokeswoman said they hoped for more information about the leaking tanks soon.

"The Department of Energy has committed to try and get us more information pretty quickly, hopefully within the next week or so. So we should have more information soon," she said.

AFP