Storage tanks holding 68 million gallons of nuclear waste in Hanford in the US state of Washington are corroding faster than anticipated, and could soon lead to soil contamination, a US Government Accountability Office (GAO) report has revealed.

"Given the current condition of the tanks, it is unclear how long they can safely store the waste," the report, released Tuesday said, adding that limited storage space is "constraining US Department of Energy's ability to respond to potential future leaks and protect human health and the environment".

The 177 storage tanks store waste from the US nuclear weapons program and are kept deep underground in Hanford.

The US Department of Energy is constructing a waste treatment plant to move the waste from storage tanks for treatment for long-term disposal. However, according to GAO, delays in constructing the treatment plant "will affect the schedule for removing waste from the tanks".

"The Department of Energy's current schedule for managing the tank waste does not consider the worsening conditions of the tanks or the delays in the construction of the Waste Treatment and Immobilization Plant (WTP)," the GAO report said, adding that the department "cannot estimate how long the waste will remain in the aging tanks".

The Department of Energy has been under pressure to ensure the safety of the Hanford nuclear site after several inspections over the past four years found corroding tanks and nuclear waste leaking into the ground.

One tank was found to be leaking 640 gallons annually, with soil contamination having started in 2010.