Japan should play a greater role in global security, including reinterpreting its pacifist constitution to allow it to defend an ally if attacked, the outgoing US ambassador said Wednesday.

Thomas Schieffer, wrapping up his tenure in Tokyo as US president-elect Barack Obama prepares to take over the US administration, said a "redefinition would be appropriate" of the post-World War II US-Japan security alliance.

"I think we ought to talk about it and we ought to try to understand where we want the alliance to go in the years ahead. I think America would welcome an alliance of equals, and I think Japan would too," he told a news conference.

"But an alliance of equals is one that has equal responsibilities and equal shares in the future," Schieffer said. "I think Japan can speak with a louder voice in international affairs."

Japan renounced the right to wage war under its US-imposed 1947 constitution and neighbouring Asian nations have sometimes looked with unease at Japanese leaders' moves away from absolute pacifism.

The conservative ruling Liberal Democratic Party has been studying whether to allow "collective self-defence," under which Japan could defend an ally in need.

Under Japan's current interpretation, it is allowed to exercise force only if directly attacked.

"I think the interpretation of collective defence needs to be looked at, as to what Japan believes that is, in the modern day with the modern kind of technology that we have," Schieffer said.

If a Japanese destroyer failed to eliminate a missile launched from Asia on the basis that it was headed for the US, "I think the American people would find that very difficult to understand the value of the alliance with Japan."

Schieffer reiterated that the United States is committed to defending Japan, where more than 40,000 US troops are deployed. North Korea fired a missile over Japan's main island in 1998.

But the alliance has recently seen strains after Japan's rising opposition, which controls one house of parliament, temporarily blocked the extension of a naval mission supplying fuel to US-led forces fighting in Afghanistan.

Japanese conservatives have also been uneasy with US concessions to North Korea under a denuclearisation pact.