Japan on Friday launched a probe into alleged decades-old secret military pacts with the United States, including one allowing US nuclear-armed ships to visit despite Tokyo's ban on atomic weapons.

Any admission of their existence would mark a landmark reversal by Japan's new centre-left government of the stance of previous conservative administrations.

Prime Minister Yukio Hatoyama's government has instructed a team of 15 people to examine more than 3,200 files at Japan's foreign ministry as well as about 400 files at its embassy in the United States.

The team will report its findings to Foreign Minister Katsuya Okada by the end of November, his ministry said.

"I would like to reveal all the things" related to the issue, Okada told journalists Thursday during a visit to New York, Kyodo News reported.

Previous conservative administrations -- including the government of Hatoyama's predecessor Taro Aso -- have denied such pacts were struck, even though US documents and former Japanese diplomats have indicated they existed.

The issue is sensitive in Japan, the only country to have suffered nuclear attacks when bombed by the United States towards the end of World War II.

Since then it has relied heavily on its alliance with the United States for its defence, including nuclear deterrence.

In 1968 Japan adopted a "three non-nuclear principles" policy of not possessing or producing nuclear weapons or allowing them on its territory, and it regularly speaks out in favour of a nuclear weapons-free world.

In 1991 US president George H.W. Bush announced that US vessels would no longer carry tactical atomic arms, rendering any pact with Japan allowing US nuclear-armed ships to visit obsolete.

Other pacts allegedly relate to a possible contingency on the Korean peninsula, the transportation of nuclear weapons to southern Japan in the event of an emergency and an agreement for Japan to pay costs linked to returning southern Okinawa to Japan in 1972 after US military occupation.