The Japanese government recently looked into the possibility of developing nuclear warhead, a news report said Monday, citing an internal government document. Yasuhisa Shiozaki, the government's top spokesman, however, denied the existence of such a document. The Japanese daily Sankei reported that experts at government organizations concluded that it would be impossible for Japan to develop nuclear weapons within a year or two, and that it would take at least three to five years to make a prototype nuclear warhead. The experts also estimated that Japan would need to spend about 200 billion yen (US$1.68 billion; â‚¬1.27 billion) to 300 billion yen (US$2.52 billion; â‚¬1.91 billion) and mobilize several hundred engineers to produce a prototype nuclear warhead, according to Sankei. The experts did not say whether Japan should develop nuclear weapons, according to the summary of the document titled "On the Possibility of Developing Nuclear Weapons Domestically," dated Sept. 20 and carried by the paper. As the only country ever attacked by atomic weapons, Japan has for decades espoused a strict policy of not possessing, developing or allowing the introduction of nuclear bombs on its territory. The non-nuclear stance, however, has come under increasing scrutiny since North Korea conducted its first nuclear test on Oct. 9, which raised severe security concerns in Japan, and broader fears that a regional arms race could be triggered. Just months prior to the North's nuclear test, it test-fired several ballistic missiles capable of hitting Japan. Several politicians have suggested that Japan should at least debate going nuclear following the Oct.9 test. The government, under newly elected Prime Minister Shinzo Abe, has said the country's pacifist Constitution does not ban it from possessing nuclear weapons for self-defense, but stressed that Japan would stick to its policy of forbidding nuclear weapons on Japanese soil. Japan's huge plutonium stockpile from nuclear power stations is a major international concern, partly because it could be a target of terror attacks or could be turned into nuclear weapons. Officials at the Defense Agency could not immediately comment on Monday.