"Those debates are significant in the sense both Tokyo and Seoul appear to be flagging their options and testing reactions," he said.

"The reactions of value are not so much what comes out of Pyongyang, but rather Washington, DC. The US is most likely going to reassure each of the two of its treaty allies of the reliability of its nuclear umbrella. But it remains to be seen how much influence it really has on de facto preparations in Seoul and Tokyo."

The message is not lost in Washington.

As some security scholars such as Rory Medcalf of Australia's Lowy Institute have noted, the White House was unusually explicit as it detailed US President Barack Obama's call to Japanese Prime Minister Shinzo Abe soon after the third North Korean nuclear test on February 12.

"President Obama reaffirmed that the United States remains steadfast in its defence commitments to Japan, including the extended deterrence offered by the US nuclear umbrella," the White House statement said.

Medcalf wrote this week that the use of the words "nuclear umbrella" would usually be termed more euphemistically - "something like 'all means'."

"This is a grim reminder that, deep down, the security of Asia rests on American capability - and presumed willingness - to use nuclear threats or force in an extreme crisis," he noted.

Two years earlier, the Obama administration went to extensive efforts to take into account both Japanese and South Korean concerns as it launched cutbacks in its nuclear arsenals as part of the 2010 nuclear posture review - cuts which were eased to sooth Seoul and Tokyo. Such overt attempts at US reassurance, however, have yet to thwart a lively debate on nuclear weapons needs, particularly in Seoul.

Elements of the Japanese right-wing - including prominent members of the Liberal Democratic Party, which recently returned to power - have long pondered the need for Japan to drop its official non-nuclear policy for "defensive" purposes. The hawkishness has also been fed by recent Sino-Japanese tensions over the disputed Diaoyu islands. The long-term prospect of a nuclear Japan is in part feeding debates across conservative security elites in South Korea.

Pro-nuke lawmakers from the ruling conservative Saenuri Party played the nuclear card during a National Assembly meeting late last month amid reports of further and larger tests by North Korea.

"The only way to defend our survival would be to maintain a balance of terror that confronts nuclear with nuclear," Representative Shim Jae-chol told his counterparts, according to the .

commentator Chung Kweon-hyun, meanwhile, pointed to Japanese advances as well as domestic surveys showing increased support for a South Korean nuclear programme.

"South Koreans must have the courage to publicly discuss the prospect of the country acquiring nuclear weapons," he wrote. "This will give it strength over the long term to suppress North Korea's and Japan's nuclear ambitions."

Such an environment has prompted the US ambassador to Seoul to take a far bolder line than the White House. Ambassador Sung Kim warned that South Korea's steps towards developing its own nuclear weapons - or the US putting back in place the tactical nuclear weapons it removed from South Korea in 1991 - would be a "huge mistake".

Underpinning the hot talk, of course, is the fact that the conceptual thinking behind nuclear weapons development is hardly foreign to South Korea and Japan. Both have a technologically advanced industrial base and both have established nuclear energy programmes. Japan, too, has stocks of weapons grade plutonium - stocks only due to rise when a controversial plant that re-processes spent uranium at Rokkasho comes into production later this year.

Japanese officials privately acknowledge a surge in questions from both foreign diplomats and local legislators about Japan's latent weapons potential. They find themselves parroting the conventional wisdom that, theoretically, Japan could probably develop such a capability within a year.

However, many analysts have long noted that far more time would be needed to turn a basic weapon into a credible, longer-term threat - including the potential need for entirely new fleets of ballistic missile submarines and strategic bombers.

Then there are the strict safeguards of the International Atomic Energy Agency on its current domestic energy facilities, including Rokkasho.

As a survey for the non-partisan US Congressional Research Service stated in a 2009 report on Japan's nuclear potential: "Japan seems to have intentionally built its nuclear programme so it would not be ideal for military use, in compliance with Japanese law."

South Korea would be starting from a slightly lower base. Just as Japan reportedly embarked on a clandestine study of nuclear weapons development in the 1960s, South Korea set up its own secret programme in the mid-1970s under the iron-fisted presidency of Park Chung-hee.

A 2011 survey of de-classified CIA documents published in Global Asia magazine suggest Park's effort was more extensive than was previously thought, running for two years longer than the US had believed. South Korea was particularly active in the period from 1976 to 1978 - a time of considerable uncertainty amid North Korean aggression - including the assassination of Park's wife - and doubts about the commitment of then-US President Jimmy Carter. It was eventually shut down following Park's assassination in 1979.

Beyond the technology, of course, is the immense political and diplomatic baggage. Any such move - secret or otherwise - would be laced with difficult ironies. South Korea and, particularly, Japan are key players in the international nuclear non-proliferation efforts.

South Korea hosted the 2012 Nuclear Security Summit and preaches the need for a de-nuclearised Korean peninsula while Japan has, of course, a pacifist constitution and its once-vaunted "Three Non-Nuclear Principles" - a pledge against the manufacture, possession or importation of nuclear weapons. Its moral position is buttressed by the fact it is the only nation to suffer a nuclear attack - the bombings of Hiroshima and Nagasaki at the end of the second world war.

There would also be considerable risks.

The authors of the Global Asia report, Peter Hayes and Moon Chung-in, warn that keeping a weapons programme truly secret proved a failure in the 1970s and would be much more difficult now.

"South Koran proliferation today would make it far more difficult to negotiate the de-nuclearisation of North Korea. An inter-Korean nuclear arms race would almost certainly lead to a new Cold War in the region involving China and Japan."

Dr Michael Green, a former White House adviser on Asia and now an expert at Washington's independent Centre for Strategic and International Studies, addressed the dangers this week.

"Both countries have the technology to have a weapon in a short time," he said of Japan and South Korea. "But not without the US finding out and not without enormous risk to their overall security situation."