Japan could arm itself with nuclear weapons in 10 years to cope with threats from China and North Korea, a study commissioned by the US Defense Department showed, according to a news report.

The report produced for the Pentagon's Office of Net Assessment (ONA) assesses that Japan can quickly build a strategic arsenal of land-based and submarine-launched missiles capable of killing up to 30 million Chinese in a nuclear war, according to the Washington Free Beacon.

"The report reveals Japan's government could arm itself with nuclear weapons within a 10-year period, based on Tokyo's advanced nuclear power infrastructure and its present day space launchers, cruise missiles, and submarines," the Washington Free Beacon said.

The ONA study was quoted as saying that the Pentagon's interest in Japan's nuclear options has taken on urgency because Japan fears that current U.S. nuclear security guarantees against Chinese or North Korean nuclear attacks are weakening and could be insufficient in the future to deter Beijing or Pyongyang.

The study also found that Japanese naval forces would likely choose a nuclear force built around off-shore based, nuclear-missile firing submarines, with a land-based component of road-mobile missiles, according to the Washington Free Beacon.

In a nuclear exchange with China, Beijing would likely strike the 20 to 30 largest Japanese cities with 3 megaton weapons, killing between 23 million and 33 million people, according to the study.

A nuclear-armed Japan could launch ballistic and cruise missiles in several retaliatory scenarios, including 150 kiloton bombings of 45 cities, killing 20 million Chinese, or the use of larger 1.2 megaton warheads on 60 cities, for an estimated toll of between 96 million and 128 million people, the study showed.

For North Korea, an attack on 10 Japanese cities with 10 kiloton warheads would kill around 1 million, and could invite a Japanese 1.2 megaton blast over the North Korean capital of Pyongyang, killing an estimated 1.1 million North Koreans, the study showed, according to the report. (Yonhap)