TOKYO, August 4, 2008 (AFP) - A US nuclear-powered submarine arrived Monday at a port in southern Japan amid protests over a radioactive leak by another vessel earlier this year.

Dozens of atomic bomb survivors and peace activists gathered in Nagasaki, one of two Japanese cities devastated by atomic bombs in World War II, bearing banners reading: 'Nuclear vessels are not welcome in Nagasaki prefecture.'

They urged the local authorities to end visits by nuclear-powered submarines until safety can be assured, said Hiroshi Sakamoto, a local peace activist.

The Pentagon said Friday that a small radioactive leak may have come from the nuclear-powered USS Houston during a cruise that included stops in Japan and Guam. The submarine docked in Sasebo City in Nagasaki prefecture in March.

A US Navy investigation determined the amount of radioactivity that seeped from a valve was less than half a microcurie, or less than what would be found in a 50-pound (22.6 kilogram) bag of lawn fertiliser.

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, activists said.

'We will continue the sit-in protest until the nuclear-powered ship leaves Sasebo port,' said Sakamoto. It was unclear how long the submarine would stay.

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