Reports say North Korea may have completed fueling a missile in advance of such a test. In 1998, North Korea fired an earlier version of its Taepodong missile over northern Japan into the Pacific Ocean.
U.S. Secretary of State Condoleezza Rice said a missile launch would be a "very serious matter" and a "provocative act."
Earlier today Japanese Prime Minister Junichiro Koizumi said Tokyo would first consult with the United States and then take what he called "severe action" if North Korea conducts the test.
Koizumi declined to offer details of Japan's possible response to a missile test.
U.S. Ambassador to Japan Thomas Schieffer said Washington could consider sanctions against North Korea, and could support action by the UN Security Council, if Pyongyang persists.
(AFP, AP, Reuters)