Japan's left-leaning government commissioned a report that confirmed longstanding suspicions that previous conservative administrations turned a blind eye to the US arms despite Japan's staunch anti-nuclear stand.

"We do not discuss the presence or absence of nuclear weapons aboard specific ships, submarines or aircraft," Pentagon spokesman Bryan Whitman told AFP.

"The US government understands the special sentiment of the Japanese people with regard to nuclear weapons and has faithfully honored its obligations under the Treaty of Mutual Cooperation and Security and will continue to do so," he said.

Japan is the only nation to have been attacked with nuclear weapons. The United States dropped atomic bombs on Hiroshima and Nagasaki in 1945, killing more than 210,000 people and leading to Japan's surrender in World War II.

Japan has since campaigned to abolish the weapons. Former prime minister Eisaku Sato won the Nobel Peace Prize largely for his "three principles" -- that Japan will not possess, produce or allow nuclear weapons on its soil.

But a panel of historians said Tuesday that Japan nonetheless allowed US warships to carry nuclear weapons across Japanese territory and, in the case of emergency, to take them to US bases on the southern island of Okinawa.

Foreign Minister Katsuya Okada said he doubted this has happened since 1991, when the United States announced the withdrawal of tactical nuclear arms from its warships.

The study was commissioned by Prime Minister Yukio Hatoyama, whose center-left Democratic Party of Japan in August ended more than a half-century of nearly unbroken rule by the conservative Liberal Democratic Party.

The United States stations some 47,000 troops in Japan as part of a security alliance reached after World War II, when Tokyo was stripped of its right to a military.

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