No nation has suffered more in the nuclear age than Japan, where atomic bombs flattened two cities in World War II and three reactors melted down at Fukushima just three years ago.

But government officials and proliferation experts say Japan is happy to let neighbors like China and North Korea believe it is part of the nuclear club, because it has a “bomb in the basement” -– the material and the means to produce nuclear weapons within six months, according to some estimates. And with tensions rising in the region, China’s belief in the “bomb in the basement” is strong enough that it has demanded Japan get rid of its massive stockpile of plutonium and drop plans to open a new breeder reactor this fall.

Japan signed the international Nuclear Non-Proliferation Treaty, which bans it from developing nuclear weapons, more than 40 years ago. But according to a senior Japanese government official deeply involved in the country’s nuclear energy program, Japan has been able to build nuclear weapons ever since it launched a plutonium breeder reactor and a uranium enrichment plant 30 years ago.

Related Story: Japan Producing Huge Stockpile of Plutonium

“Japan already has the technical capability, and has had it since the 1980s,” said the official. He said that once Japan had more than five to 10 kilograms of plutonium, the amount needed for a single weapon, it had “already gone over the threshold,” and had a nuclear deterrent.

Japan now has 9 tons of plutonium stockpiled at several locations in Japan and another 35 tons stored in France and the U.K. The material is enough to create 5,000 nuclear bombs. The country also has 1.2 tons of enriched uranium.

Technical ability doesn’t equate to a bomb, but experts suggest getting from raw plutonium to a nuclear weapon could take as little as six months after the political decision to go forward. A senior U.S. official familiar with Japanese nuclear strategy said the six-month figure for a country with Japan’s advanced nuclear engineering infrastructure was not out of the ballpark, and no expert gave an estimate of more than two years.

In fact, many of Japan’s conservative politicians have long supported Japan’s nuclear power program because of its military potential. “The hawks love nuclear weapons, so they like the nuclear power program as the best they can do,” said Jeffrey Lewis, director of the East Asia Non-Proliferation Program at the Monterey Institute of International Studies in California. “They don’t want to give up the idea they have, to use it as a deterrent.”

Many experts now see statements by Japanese politicians about the potential military use of the nation’s nuclear stores as part of the “bomb in the basement” strategy, at least as much about celebrating Japan’s abilities and keeping its neighbors guessing as actually building weapons.

But pressure has been growing on Japan to dump some of the trappings of its deterrent regardless. The U.S. wants Japan to return 331 kilos of weapons grade plutonium – enough for between 40 and 50 weapons – that it supplied during the Cold War. Japan and the U.S. are expected to sign a deal for the return at a nuclear security summit next week in the Netherlands.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Yet Japan is sending mixed signals. It also has plans to open a new fast-breeder plutonium reactor in Rokkasho in October. The reactor would be able to produce 8 tons of plutonium a year, or enough for 1,000 Nagasaki-sized weapons.

China seems to take the basement bomb seriously. It has taken advantage of the publicity over the pending return of the 331 kilos to ask that Japan dispose of its larger stockpile of plutonium, and keep the new Rokkasho plant off-line. Chinese officials have argued that Rokkasho was launched when Japan had ambitious plans to use plutonium as fuel for a whole new generation of reactors, but that those plans are on hold post-Fukushima and the plutonium no longer has a peacetime use.

In February, the official Chinese news agency Xinhua published a commentary that said if a country "hoards far more nuclear materials than it needs, including a massive amount of weapons grade plutonium, the world has good reason to ask why."

"If you were distrustful, then you see it through a different lens."

Steve Fetter, formerly the Obama White House’s assistant director of the Office of Science and Technology Policy, thinks China's concerns are not purely political.

"I've had private discussions with China in which they ask, 'Why does Japan have all this plutonium that they have no possible use for?' I say they made have made a mistake and are left with a huge stockpile," said Fetter, now a professor at the University of Maryland. "But if you were distrustful, then you see it through a different lens."

For at least four or five years, said Leonard Spector, deputy director of the Center for Non-Proliferation Studies in Monterey, the Japanese plutonium stockpile has been mentioned as a threat in Chinese defense white papers.

Japan, of course, has its own security concerns with China and North Korea. North Korea's nuclear weapons program is a direct threat to Japan. Some of its Nodong missiles, with a range capability of hitting anywhere in Japan, are believed to be nuclear-armed. "Nodong is a Japan weapon," said Spector.

There have been confrontations between China and Japan over small islands north of Taiwan. The dispute has recently escalated. In October, state-controlled media in China warned "a war looms following Japan's radical provocation," Tokyo's threat to shoot down Chinese drones.

Most experts agree that China is the greater threat, because as one expert said, "If North Korea attacked Japan, the U.S. would flatten it"-- and thus China is the country Japanese officials, particularly the right, want to impress with their minimal deterrence.

But experts also note that another nation in the region seems to have been impressed by the Japanese “bomb in the basement” strategy, not as a threat but as a model.

"China and South Korea will use this as an excuse, each in their own way."

There are fears that if Japan opens the Rakkosho plant, it will encourage South Korea to go the same route as its neighbor. The U.S. and South Korea have been negotiating a new civilian nuclear cooperation pact. The South wants to reprocess plutonium, but the U.S. is resisting providing cooperation or U.S. nuclear materials.

Jeffrey Lewis believes that the South Koreans want to emulate Japan, and says there is a “bigger bomb constituency in South Korea , about 10 to 20 percent [of the population],” than in Japan.

"The least of my concerns is that Japan would get a nuclear weapon," said Fetter. "But China and South Korea will use this as an excuse, each in their own way."

And, in fact, not everyone believes that Japan COULD go all the way. Jacques Hymans, a professor of international relations at the University of Southern California, believes the process would be thwarted by what he calls "veto players," that is, government officials who would resist a secret program and reveal it before it reached fruition. He wrote recently that Japan has more levels of nuclear bureaucracy than it once had, as well as more potential “veto players” inside that bureaucracy because of Fukushima. He said that any attempt to make a bomb would be "swamped by the intrusion of other powerful actors with very different motivations."

Still, even without a bomb, Japan has achieved a level of nuclear deterrence without building a bomb and suffering sanctions. That may be a more impressive achievement than actually building a bomb.

Follow NBC News Investigations on Twitter and Facebook