Defense Minister Fumio Kyuma said Japan's currently planned missile defense system is designed to intercept medium-altitude missiles, and may not be able to take down ones flying at higher altitudes. Generally, the greater a missile's range, the higher it must fly to reach its target.
A South Korean news report last month said North Korea had displayed at a recent military parade a newly developed medium-range ballistic missile capable of reaching the U.S. territory of Guam.
Kyuma was questioned at a news conference Tuesday about the suspected North Korean missile, which reportedly can travel 2,500-4,000 kilometers (1,500-2,500 miles).
He wouldn't confirm its existence, but said both Japan and the United States need to study ways of countering the potential threat.
Kyuma said the interceptor missiles that Japan currently plans to deploy, called Standard Missile-3 (SM-3), will target missiles flying at a medium altitude, so "technological research on those flown at high altitude will also be necessary, even for the United States," according to a transcript of his comments published on the ministry's Web site.
Japan deployed its first advanced U.S.-developed Patriot missiles this year, and plans to introduce SM-3 interceptors on its destroyers over the next few years.
Japanese people was stunned in 1998 when North Korea launched a missile over Japan, and in July 2006 when it test-fired a series of other missiles that landed in the Sea of Japan.
North Korea also exploded an underground nuclear device in October, further boosting worries among Japanese.
About 50,000 U.S. military personnel are stationed in Japan under a mutual security arrangement.