A group of former US policymakers called Wednesday for Japan to expand the scope of defense cooperation with Washington, saying that the two nations' alliance needed momentum to overcome strains.

The study group, set up under the Center of Strategic and International Studies think tank, does not represent US policy but similar reports have in the past been used as a basis for government planning.

The report warned of "a time of drift" between the United States and Japan, saying that the six-decade alliance between the new countries is being reshaped by developments including the rapid rise of China.

"As leaders in both the United States and Japan face a myriad of other challenges, the health and welfare of one of the world's most important alliances is endangered," it said.

"A stronger and more equal alliance is required to adequately address these and other great issues of the day," it said.

The study said that Japan should loosen its prohibition on so-called "collective self-defense" to allow military cooperation with the United States, saying that existing restrictions were "an impediment to the alliance."

Under the 1947 constitution imposed by the United States, Japan forever renounces the right to wage war. Japan nonetheless maintains armed forces and has long debated to what extent it can operate with the United States.

Wednesday's report noted that Japanese and US forces worked together after the March 11, 2011 tsunami disaster and said that the two nations should address the "irony" that they cannot fight together against an external threat.

The study also called on Japan, which has participated for two decades in UN missions, to extend the legal latitude for its peacekeepers to allow them to use force if needed to protect other nations' troops.

In concrete terms, the report said Japan should increase surveillance of the tense South China Sea alongside the United States and be prepared to send minesweepers to the Middle East if Iran indicates it will close the strategic Strait of Hormuz.

The study was led by Richard Armitage, a deputy secretary of state under president George W. Bush, and Joseph Nye, a professor at Harvard University who served in positions under presidents Jimmy Carter and Bill Clinton.