Japan's new policy on military action would allow its forces to come to the aid of a US naval ship under attack, Tokyo's defense minister said Friday.

In a visit to Washington, Itsunori Onodera cited the hypothetical scenario as he sought to explain the Japanese government's controversial decision to ease decades-long restrictions on the country's military.

If US warships were sent to defend Japan, and those ships were attacked, the Japanese "constitution was interpreted to say we could not help that ship," Onodera told an audience at the Center for Strategic and International Studies, a Washington think tank.

But taking action to assist an ally was "what an ally should naturally do," he said through an interpreter. "That's how this change in policy should be understood."

Onodera said the change approved by Prime Minister Shinzo Abe's cabinet on July 1 would bolster Tokyo's alliance with the United States, opening the way to new forms of military cooperation.

"We believe this will dramatically deepen our ties with the United States," Onodera said.

Japan's decision to reinterpret its pacificist constitution has provoked anger at home as well as among its neighbors, with China expressing outrage and alarm.

The ground-breaking shift has come against the backdrop of soaring regional tensions with China over disputed islands.

But the United States has endorsed the change and at an earlier joint press conference at the Pentagon, Defense Secretary Chuck Hagel praised Tokyo's move.

"This bold, historic, landmark decision will enable Japan to significantly increase its contribution to regional and global security and expand its role on the world stage," Hagel said.

Washington has long encouraged Japan to go ahead with the change to permit Tokyo to share more of the burden in what has been a lopsided defense relationship.

The Japanese minister sought to counter criticism of the shift in policy, arguing it would enable Tokyo to better protect its population and deter potential adversaries.

Onodera also said Japan had increased defense spending for the first time in years and was improving its "world-class" missile defense system, setting up amphibious units and strengthening its "maritime forces" to "protect our islands."

The rise in defense spending has been seen as a bid to counter China's growing military muscle and assertive stance on territorial claims.

Onodera said Japan was always open to dialogue with China but if faced with "unilateral" actions, "we must respond firmly."