North Korea's announcement yesterday that it has tested a nuclear bomb is set to push Japan to expand its own military and stir debate on what was once the ultimate taboo of developing atomic weapons itself.

The test comes with Japan in the midst of expanding its defence posture, 60 years after it was defeated in World War II and forced by the United States to renounce the right to a military.

Prime Minister Shinzo Abe, who took office just two weeks ago, is a sworn hawk on North Korea who has long supported a larger role for Japan's military alongside its ally the United States.

"We need to make a stern response and North Korea will be responsible for all the consequences," Abe said Monday as he visited Seoul "Japan for its part will immediately start studying a response with stern measures."

Analysts expect North Korea's test to boost the hand of Abe, who wants to rewrite the pacifist 1947

constitution and allow Japanese troops to engage in overseas operations alongside allies.

Washington currently protects Japan by treaty as the country was stripped of its right to maintain an armed forces after defeat in World War II.

But after North Korea in 1998 fired a missile over Japan's main island, Japan and the United States started working in earnest on a missile shield.

Abe said Monday that Japan would step up cooperation with the United States, including on missile defence, "to maintain the safety of the Japanese country and people."

The United States stationed its first surface-to-air Patriot missiles in Japan after North Korea in July test-fired seven missiles in Japan's direction.

Despite its pacifism and US guarantees to protect Japan, the country now has around 240,000 troops on active duty and an annual military budget of 4.81 trillion yen (41.6 billion dollars).