A statement from the US government on Thursday said the Houston had been leaking radiation from June 2006 to July 2008.

During that time, the Houston docked at the Japanese ports of Yokosuka, 45 km (30 miles) southwest of Tokyo and in the southern island of Okinawa, as well as at Sasebo, 980 km (610 miles) southwest of Tokyo, the US statement said.

Both the US and Japanese governments said the radiation leak was too small to cause harm.

"We do not think that the amount of leakage would have any impact on humans or the environment," a Japanese foreign ministry official said.

The Houston may have also released a small amount of radioactivity into Pearl Harbor in Hawaii and Guam, the US statement said.

The radiation leak is a fresh blow for Tokyo and Washington, which has been planning to station a nuclear-powered aircraft carrier in Japan, the only nation in the world to have suffered nuclear attacks.

Local residents and civic groups expressed concern over the deployment of the USS George Washington after a fire on the nuclear-powered warship in May. They called for more information about that fire.

Japan said the Houston's radiation leak would not have any impact on the plan to deploy the George Washington at Yokosuka.

"The United States assures strict procedures and prevention systems for nuclear-powered warships coming into port, and Japan is also checking the radiation levels 24 hours a day," the foreign ministry official said.

The Houston radiation leak caused a big media stir in Japan last week, with the foreign ministry criticised for failing to disclose the leak promptly to the government and the public. (Reporting by Yoko Kubota; Editing by Paul Tait)