TOKYO (AP)  U.S. officials have found that an American nuclear submarine leaked radiation for more than two years and may have affected Japanese ports more extensively than initially thought, Japan's government said Thursday. The Foreign Ministry said a report provided by the U.S. Embassy showed that the USS Houston "has been steadily leaking a small amount" of radiation from June 2006 until last month, when it entered a dry dock in Hawaii. Last week, the U.S. Navy notified Japan that the submarine may have leaked a small amount of radiation during three visits to the southern Japanese ports of Sasebo and Okinawa in March and April this year, but the amount caused no threat to humans or the environment. It said at the time that slightly radioactive water may have been slowly leaking from a valve since March as the Los Angeles-class submarine traveled around the Pacific. According to the U.S. Embassy report, provided by the ministry, the submarine was also leaking during nine earlier port calls in Japan. It said the submarine made three previously undisclosed calls at Sasebo in July 2006 and in February and April 2007, two port calls in 2007 in Yokosuka, a large U.S. naval base near Tokyo, and four in Okinawa in 2007. The report said the total leakage was still negligible and was below the limit of detection by personnel on the ship. Embassy officials were not available for comment Thursday evening. The radiation leak has caused a stir in Japan, where the continued presence of the U.S. military and its nuclear vessels remains controversial. News of the incident also comes just weeks ahead of the arrival of the nuclear-powered aircraft carrier USS George Washington to be based in Yokosuka, just south of Tokyo. The carrier's arrival, originally set for August under a Japan-U.S. security alliance, was delayed until late September because of a fire aboard the vessel in May, another incident that has caused safety concerns in Japan. Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed. Guidelines: You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. Read more