Senior officials have called for Japan to discuss the nuclear option in the face of the threat from communist neighbor North Korea, which said on October 9 it had tested its first atomic bomb.

Japan, the only nation to be attacked with atomic bombs, has a four-decade policy against the possession, production and presence of nuclear weapons on its soil.

"The United States also understands very well the three nuclear principles here in Japan and they are not inconsistent with American foreign policy goals here," US Ambassador to Japan Thomas Schieffer told reporters.

"From our standpoint, we have been able to work under those guidelines for a long time and we see no necessity for changing that today," he said.

Prime Minister Shinzo Abe, known for his passionate support of a larger military role for Japan, has ruled out developing nuclear weapons.

But one of his top policy aides, Shoichi Nakagawa, and Foreign Minister Taro Aso have said Japan needed at least to debate the nuclear option, in light of North Korea.

Schieffer said Washington had no objections to the debate in Japan, one of its closest allies.

"What the Japanese talk about with themselves or with their government is up to the Japanese. It is not up to the United States to decide what is appropriate or not appropriate for the Japanese to say," Schieffer said.

Abe said Friday that individual lawmakers were free to express their opinions, even though the government and his ruling Liberal Democratic Party would not take up the issue.

"It is clear that it will not be discussed by the government or a formal party organ," Abe told a meeting of newspaper editors.

But he added: "Other than that, discussions cannot be suppressed because Japan is a free country."

Former prime minister Eisaku Sato proposed developing nuclear weapons in the 1960s, as China built the bomb, but dropped the plan in the face of objections from the US.

Sato later declared the three non-nuclear principles and won the Nobel Peace Prize.

The United States destroyed the cities of Hiroshima and Nagasaki at the end of World War II, in the world's only atomic attacks. More than 210,000 people died instantly or from horrific burns.

The US forced Japan to renounce the right to wage war after its defeat, and has since provided it with a security umbrella.

Abe wants to revise the US-imposed constitution's Article Nine, under which Japan renounced the right to maintain a military or even threaten to use force.

Such changes are viewed with unease in China and the two Koreas, which remain resentful of Japan's past aggression.

Schieffer said the US did not have concerns about constitutional revision, a process expected to take several years.

"I don't know that there is an overriding concern that we have about Article Nine," Schieffer said.

"I don't think revision of Article Nine would stand in the way of us being able to do things together for our mutual benefit," he said.

Despite its official pacifism, Japan has about 240,000 troops on active duty and an annual military budget of more than 41 billion dollars, the fourth-highest in the world.

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