The U.S. Navy says trace amounts of radiation detected near one of its nuclear submarines at a Japanese port are no cause for alarm.

The U.S. Navy reacted quickly Thursday to allay concerns about a radiation leak from one of its submarines in Japan.

The Japanese government said Wednesday that tiny amounts of radiation had been detected from the U.S.S. Honolulu earlier this month during the submarine's final deployment at Yokosuka. The government says this is the first time since such readings were initiated in 1964 that radiation from an American vessel in Japanese waters has been detected.

U.S. Naval Forces Japan spokesman Jon Nylander says the Navy is conducting an investigation to verify the leak, but the reported radiation levels are not a threat to people or the environment.

"The concentration of radioactivity reported in the water sample is about 5,000 times less than the naturally occurring radioactivity found in sea water throughout the world," he said. "The concentration is a factor of 1,000 below the clean drinking water concentrations for the public in the U.S. and should be of no concern to the public or regulatory officials."

But Japan's anti-nuclear Citizens' Nuclear Information Center said Thursday that even though the detected radiation is a small amount, it is still "very serious."

The U.S.S Honolulu is based at Pearl Harbor in Hawaii. Like other U.S. nuclear submarines it makes port calls in Japan, where about 50,000 American military personnel are stationed.

The Honolulu, a fast-attack submarine launched in 1983, is currently docked at Saipan, a U.S. island in the western Pacific. The submarine is due to be decommissioned in the U.S. next month.

The radiation revelation comes as the city of Yokosuka prepares to permanently host a U.S. nuclear-powered aircraft carrier. Despite local opposition because of safety concerns, the city's mayor agreed earlier this year to allow the U.S.S. George Washington to be based at Yokosuka from 2008.

Japan has more than 50 nuclear-power plants but nuclear weapons and the presence of nuclear-powered American naval vessels remain sensitive issues here.

Japan is the only country to be a victim of atomic attacks. The United States dropped atomic bombs on Hiroshima and Nagasaki in August 1945 prompting Japan's surrender in the Second World War.