The US says the amount of radioactive water leaked was negligible, but the news has caused a stir in Japan, where both the US military presence and its nuclear vessels are controversial.

An investigation showed water may have been slowly leaking from the nuclear-powered USS Houston since March as it travelled around the Pacific.

But the Japanese Foreign Ministry said it was not known how long the Los Angeles-class submarine had been leaking the cooling water.

It made port calls in the southern Japanese naval ports of Sasebo and Okinawa in March and April.

Sasebo city official Akihiro Yoshida said government monitoring during the submarine's calls showed no abnormal increase of radioactivity in the area's waters at the time.

'Still, we are rather concerned,' he said.

The Pentagon said on Friday that the total amount of radioactivity released into the environment from the USS Houston at each stop was less than half a microcurie - a negligible amount equivalent to the radioactivity of a 22kg bag of fertiliser.

The leak was found during an inspection of the USS Houston in Hawaii last month.

The incident comes at a time when the US has been trying to allay fears over the planned stationing of nuclear-powered aircraft carrier George Washington in Japan, the only country to have suffered nuclear attacks.

A civic group said the leak raised doubts about the safety of the US Navy's nuclear-powered vessels.

A fire broke out on board the George Washington in May, adding to concerns about the ship's safety.

The US Navy replaced the George Washington's captain after the incident.

The aircraft carrier is scheduled to arrive in Yokosuka, 45km south-west of Tokyo, late next month.

News of the leak was also an embarrassment for the new Japanese Cabinet, which was sworn in yesterday, as media reports had preceded the official announcement yesterday morning.

Chief Cabinet Secretary Nobutaka Machimura, the government's No. 2 who retained his post in the Cabinet, criticised the Foreign Ministry for the delay.

Defence Minister Yoshimasa Hayashi said: 'We'd have liked to hear from the Foreign Ministry earlier.'

Foreign Minister Masahiko Komura, who also retained his post, learnt of the incident on television yesterday, a day after the US Navy informed the Japanese side.

'This morning, I was watching CNN, and even if I don't understand English that well, I saw that something strange was going on,' he said.

AP, AFP, Reuters