Tokyo -- The prospect of a nuclear-armed Japan is at once unimaginable and obvious.

Hard to believe, because the only country ever bombed with atomic weapons carries a survivors' burden to make the moral case against using them again.

Yet a natural development, because North Korea's recent accession to the nuclear club leaves Japan facing an arc of nuclear-armed countries with whom it has testy relations.

So it was perhaps inevitable in the wake of the North Korean nuclear test that a senior Japanese government official would openly muse about the prospect of Japan acquiring a nuclear deterrent of its own.

"We need to find a way to prevent Japan from coming under attack," Shoichi Nakagawa, chairman of the governing Liberal Democratic Party's policy council, told a weekend TV talk show. "There is an argument that possession of nuclear weapons is one such option."

His remarks rippled through nervous capitals, with US President Bush warning that China was "deeply concerned" about the advent of a regional arms race. Secretary of State Condoleezza Rice alluded to the dangers of a building momentum for other countries to go nuclear, warning en route to Tokyo on Tuesday that "an event of this kind does carry with it the potential for instability in the relationships that now exist in the region."

Other Bush administration officials have made the case this week that Japan, like South Korea, is already protected by the US nuclear umbrella.

"Why would Japan be more secure if it had nuclear weapons?" asked J. Thomas Schieffer, America's ambassador to Japan. "If somehow we were at this point in time and the alliance was wobbly, you'd have a whole different context. But the alliance has never been stronger."

Nakagawa swiftly restated his position, saying he did not favor a nuclear Japan and had only been calling for a debate on all security options. Japanese Prime Minister Shinzo Abe reaffirmed his pledge that Tokyo would abide by the three principles of nuclear nonproliferation that are a cornerstone of the nation's foreign policy: a pledge against building nuclear weapons, possessing them, or allowing them to be stationed on Japanese soil.

But the reasons for snuffing out any debate over a Japanese bomb have more to do with political pressures than any taboo against discussing it.

Abe wants the world to be focused on the dangers of a real North Korean bomb -- not shift its attention to the implications of a potential Japanese weapon. Japan has the necessary uranium and plutonium stocks and the technology to build a bomb, quite easily, experts say. But a Japanese bomb would also stoke the anti-Japanese embers in China and South Korea, where many profess to worry about Tokyo shucking its postwar pacifism and sense of responsibility for its imperial history.

Any internal pressure for a nuclear program comes largely from hard-line nationalists concerned that Japan cannot depend indefinitely on Washington's protection. It is a Japanese variation of the nationalist strain that pushed France into building its own nuclear arsenal. In a real nuclear exchange, the Japanese might ask, would the US really risk Los Angeles to defend Tokyo?

Many of those who argue for a Japanese bomb belong to a younger generation of politicians who have less desire to rely on US forces. "Abe is quite average among his generation in that he has less trust in the absolute dependence on US military protection," says Hideaki Kase, a conservative commentator.

Those who argue that Japan must rely on its own defenses downplay the dangers of an arms race. They contend that mutual US-Soviet nuclear deterrence kept the Cold War cold. And they say that a nuclear balance in Asia, in which South Korea and Japan would join the nuclear club that already includes China and Russia in addition to North Korea, could do the same.

But the new prime minister has tried to crush any suspicion that he favors taking Japan nuclear. Abe wants to avoid creating a hothouse atmosphere that could imperil his ambitious conservative agenda of domestic reforms. The core of his program, from rewriting the pacifist constitution to restoring patriotism and traditional Japanese values in education, is a radical challenge to the postwar order that is the legacy of the US occupation.

To pave the way, Abe has moved within his first month in office to assuage criticism that he is a hawk whose policies will lead to a renewed militarism. He made trips to Beijing and Seoul that, on the surface at least, have improved Tokyo's relations with those capitals. Keeping the Chinese relationship on track is particularly crucial to Abe, and press reports here this week said the prime minister had assured visiting Chinese officials that Japan has no intention of developing a nuclear arsenal.

The North Korean bomb offers those three countries a window to come together against a new threat, even if they disagree on how to discourage Kim Jong Il from nuclear drive. Keeping the focus on North Korea's capabilities also provides Japan with political cover to continue modernizing its military.

North Korea's summer missile launches and the nuclear test this month also effectively created a consensus that possessing a missile-defense system in partnership with Washington is a must-have option.

Missile defense has great public support, but Abe would face a much more emotional battle if he took the lid off the nuclear weapons debate.

"Public opinion won't allow nuclear weapons," says Yasuhiro Okudaira, a member of an association that seeks to preserve the pacifist part of Japan's Constitution. "We have an antinuclear consciousness that was nurtured by Article 9. Even Abe changed his thinking after becoming prime minister."