U.S. top military commander in the Pacific, Adm. Timothy Keating, left, is escorted by Japanese Defense Minister Fumio Kyuma at the Defense Ministry in Tokyo yesterday. He's in Japan to discuss the two countries' missile defense systems. Junji Kurokawa | Associated Press

The talks, during Adm. Timothy Keating's first overseas trip since taking over at the U.S. Pacific Command last month, addressed the sensitive Japanese constitutional debate over whether Tokyo may shoot down an enemy missile aimed at the United States.

The trip comes as both nations accelerate their construction of ballistic missile defense networks amid fears North Korea could fire one of its missiles at Japan or the U.S.

Tokyo has embraced missile defense like no other U.S. ally in the decade since Pyongyang unexpectedly test-fired a long-range missile over Japanese territory in 1998.

Today the program to detect and shoot down enemy missiles has become a key focal point of the U.S.-Japan alliance.

"Japan is a willing partner with us," Keating told The Associated Press en route to Japan from his Hawai'i headquarters.

He noted Japan was using similar equipment as the United States and had been sharing data from their missile detection and tracking systems with the U.S.

"All of this integrates into a very complex but elegant ballistic missile defense system for the United States and Japan," Keating said.

Yet some issues governing how the two countries work together on missile defense are unresolved, including how far Japan can go to help the United States.

The U.S. is committed to protecting Japan under a decades-old security treaty and would have little trouble firing a missile to intercept one heading for Japan.

It's unclear, however, whether Japan's war-renouncing constitution allows Tokyo to shoot down a missile if it knows the projectile is targeted at the U.S.

That's because the document prohibits the use of force except in self-defense and restricts Japan's ability to come to the aid of another nation if its own security is not threatened.

The issue has never been a problem for the two allies until now because Tokyo did not have the ability to play a serious role in the defense of the U.S. That changed when Japan acquired systems to protect itself from any possible ballistic missile attack — systems that could also track and intercept missiles aimed at the United States.

Japanese media reported last week that Prime Minister Shinzo Abe planned to create a panel that would examine easing these restraints so Tokyo could shoot down ballistic missiles aimed at the U.S.

Keating raised the issue with Japan's top uniformed officer, Adm. Takashi Saito, today while mentioning other unresolved issues governing bilateral missile defense cooperation.

"Can Japan have the authority under the constitution to launch an Aegis missile against a threat if you're not certain where this missile is going? It could be overflying Japan or Honolulu," Keating said. "It's complicated. We've got to work through all this."

Keating spoke after reviewing an honor guard with Saito in front of a row of blooming cherry trees at the Defense Ministry in Tokyo. He later met with Abe at the prime minister's office.

Keating told Saito he made sure Japan was his first stop on his inaugural trip through the Asia-Pacific region as commander.

Saito replied Keating's move "sent the right message" to Japan's neighbors about the strength of the U.S.-Japan alliance. Japan has snapped up missile defense capabilities in recent years in an aggressive acquisition of military power rarely seen in Tokyo.

A few weeks ago, Japan, acting a year earlier than originally scheduled, deployed Patriot anti-missile batteries to protect Tokyo. Last month, it acquired its fifth navy ship equipped with the Aegis system capable of tracking ballistic missiles.

Tokyo also recently overrode a three-decades-old policy against exporting weapons to enable it to help the U.S. build an interceptor missile.

The U.S. Navy, meanwhile, has deployed some of its most advanced missile defense technology on ships stationed in Japan. The U.S. Pacific Fleet is well along the way with plans to equip 18 cruisers and destroyers with the technology to track and intercept ballistic missiles by next year.

There is solid political support in Japan for ballistic missile defense. Pyongyang's firing of a long-range test missile over northern Japan in 1998 created the groundwork.

Support has only grown in the years since as North Korea withdrew from the Nuclear Non-Proliferation Treaty and tested an atomic device in October last year.

The U.S. and Japan had a chance to check how well they would respond to a missile attack last July when North Korea test-fired seven missiles into waters between the Korean peninsula and Japan.