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 .
NEW : Total radiation leaked less than that found in a smoke detector , Navy says . Navy had earlier said USS Houston leaked radioactive water for June 2006 . Leak existed since months , Navy says . USS Houston visited several Japanese ports , plus Malaysia and Malaysia .