"In a case in which Japan's security cannot be protected unless we accept temporary port calls by U.S. ships carrying nuclear weapons, the government at that time would make a decision even if it may affect its political fortunes," Okada told a parliamentary committee.

The issue of introduction of nuclear weapons into Japan has drawn renewed attention after a recent investigation conducted by a Foreign Ministry panel acknowledged that "a tacit agreement" was reached between Japan and the United States in the Cold War era which led Tokyo to allow U.S. nuclear-armed vessels to visit Japanese ports.

The move revealed that Japan's non-nuclear principles of not possessing, producing or allowing nuclear weapons on its territory were a mere facade.

But in 1991, after the Cold War ended, U.S. President George W. Bush announced he would withdraw tactical nuclear weapons from U.S. vessels, making the secret pact with Japan obsolete.