WASHINGTON (CNN) -- A U.S. Navy submarine leaked trace amounts of radioactive water for two years as it made port calls in Japan and other Asian nations, the Navy said Thursday.
The USS Houston arrives in Pearl Harbor, Hawaii, for routine maintenance, during which the leak was found.
Last week, Navy officials told Japan that the USS Houston, a Los Angeles-class fast attack submarine, had made one port call -- in March -- while leaking the contaminated fluid.
But after reviewing records of the sub, the Navy told Japanese officials Thursday that the Houston had been leaking much longer, since June 2006, and had made port calls to Japanese bases at Sasebo, Yokosuka and Okinawa before the leak was discovered.
Officials also have told the governments of Malaysia and Singapore that the sub made port calls to those countries while leaking the radioactive water, Navy officials said. The Houston also made stops in Guam and Hawaii. See a map of the submarine's travels »
A Japanese Foreign Ministry spokesman said Tokyo appreciated the United States reporting the new information promptly.
"At the same time, what remains to be nailed down is the cause," said the spokesman, Hiroshi Suzuki. "We still need to get to the bottom of the cause of this leak. We would like to get a full explanation."
Suzuki said the U.S. government has assured Japan it is taking all precautionary measures to guarantee safety. In addition, Japan also conducts round-the-clock monitoring of U.S. vessels in Japanese ports.
The U.S. Navy said Thursday that the leak was minor in both volume and radioactivity, and that the radioactivity from the leaks in all foreign ports added up to less than that found in a smoke detector.
The problem was discovered last month when a buildup of leaking water popped a covered valve and poured onto a sailor's leg while the submarine was in dry dock.
A subsequent investigation found a valve was slowly dripping water from within the nuclear power plant onboard, but it was not water that came in direct contact with the nuclear reactor, Navy officials said.
The water reportedly was so low in radiation it was barely detectable.
Officials with knowledge of the incident could not quantify the radiation level but insisted it was "negligible" and an "extremely low level." The total amount of radioactivity that leaked while the sub was in port in Guam, Japan and Hawaii was less than a half of a microcurie (0.0000005 curies), or less than what is found in a 50-pound bag of lawn and garden fertilizer, the officials said.
The sailor who was doused, a crew member of the Houston, tested negative for radiation from the water, according to Navy officials.
While Japan and the United States have come to agreements to allow nuclear-powered ships to make port calls in that country, the decision was a not a popular one in Japan.
The Houston incident comes at a time when the Navy is trying to smooth over another problem with a nuclear-powered aircraft carrier.
The USS George Washington was due to replace the aging, conventionally powered USS Kitty Hawk this summer as the sole U.S. carrier based in Japan.
While en route to Japan this May, a fire broke out on the George Washington, causing $70 million in damage. The fire was blamed on crew members smoking around improperly stored flammable materials.
There was no damage or threat to the nuclear reactor, but the ship was diverted to San Diego, California, for repairs. It is expected to arrive in Japan at the end of September.
The Navy fired the George Washington's captain and his deputy last week, saying an investigation into the fire led to a lack of confidence in the leadership of both men.
Three weeks ago, thousands of Japanese protested the pending arrival of the George Washington.
CNN's Kyung Lah contributed to this report.