The USS Houston steadily leaked a small amount of radiation from June 2006 until it was put in drydock in Hawaii last month, Japan's foreign ministry said, quoting information from the US authorities.

During that period the vessel stopped five times in Sasebo City, five times in Okinawa and once in Yokosuka, the government said.

The new incidents emerged after the Pentagon announced Friday that a small radioactive leak came from the submarine during a stop in Sasebo City in March.

The announcement sparked protests in Japan, the world's only country to have suffered nuclear attack.

A US Navy investigation determined the amount of radioactivity that seeped from a valve was just over half a microcurie, or less than what would be released in one day of normal operation of a US civilian nuclear power plant.

There was no risk to the safety of the crew, the public, marine life or the environment as a result of the port visits, US authorities said.

Another US nuclear-powered submarine, the USS La Jolla, called Monday at the same port in Sasebo City, where it was met by a protest by about 50 peace campaigners and local residents.

The port call came just days ahead of the 63rd anniversary of the US atomic bombing of Nagasaki on August 9, 1945.

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