"The amounts were very, very, very small and were not of the sort that would affect the human body or the environment," Chief Cabinet Secretary Nobutaka Machimura told a news conference.

The incident comes at a time when the United States has been trying to allay fears over the planned stationing of nuclear-powered aircraft carrier George Washington in Japan, the only country to have suffered nuclear attacks.

A civic group opposed to the deployment of the George Washington said the leak raises doubts about the safety of the US Navy's nuclear-powered vessels.

"The claim...that the US Navy's nuclear-powered vessels are safe and are not susceptible to radiation leaks has completely broken down," Masahiko Goto, a lawyer representing the group said in a statement.

Goto called on the US Navy to disclose the causes of the radiation leak and urged the Japanese government to seek complete disclosure.

Many Japanese are sensitive about the use of nuclear power by military forces. The Japanese cities of Hiroshima and Nagasaki came under nuclear attack from the United States at the end of World War Two in August 1945.

DRY DOCK

The radiation leak from the Houston came to light after an incident in July, when the submarine was in dry dock, Lieutenant Commander David Benham, spokesman for Commander Submarine Forces, Pacific in Pearl Harbor, said by telephone.

Around a gallon of water that contained a small amount of radioactivity spilled out from a pipe onto a crew member of the Houston, Benham said. But no contamination was found on the sailor, he said.

It was unclear when the radiation leak had started, and whether there had been any leak while the Houston was in Sasebo in southern Japan from late March to early April, he said.

"But seeing as we couldn't rule it out, we notified the government of Japan," Benham said.

"It was a very minute amount, and there was never any danger to the reactor of the ship or to the crew of the ship, or for that matter to the public," he added.

"We're talking about an extremely minute amount of radiation...it's about half as much radiation as you would find in a 50 pound bag of fertiliser," Benham said.

Twenty-four hour radiation monitoring conducted by the Japanese government during the Houston's stay in March and April had revealed no irregularities, the Japanese Foreign Ministry said.

The aircraft carrier George Washington is scheduled to arrive in Yokosuka, 45 km (28 miles) southwest of Tokyo, in late September.

A fire broke out on board the nuclear-powered carrier in May, adding to concerns about the ship's safety. The US Navy replaced the George Washington's captain after the incident.

(Editing by Sanjeev Miglani)