Japan and the United States will jointly develop a fuel-cell powered submarine that can run for a month under the sea on a single charge, a report says.
The top-selling Yomiuri Shimbun reported that the unmanned, 10-metre long sub would be able to chart a pre-programmed course before returning to base.
The story, citing unnamed Japanese defence ministry officials, comes as Tokyo and Washington look to beef up their security alliance as they warily eye an increasingly assertive China.
Defence ministry officials could not immediately confirm the deal.
The submarine would be used for patrolling with sonar capable of detecting potential threats, but it would not be equipped with torpedoes or other weaponry, the Yomiuri said.
Japan's defence ministry would earmark about 2.6 billion yen ($A27 million) over the next five years to develop the high-performance fuel cell, it added.
The US military reportedly got involved when it heard about its Japanese counterparts' plans for a fuel-cell sub.
Fuel cells generate emissions-free energy through a chemical reaction of hydrogen and oxygen, and are most commonly associated with environmentally friendly vehicles.
Japan is a leader in the technology while the US a major player in hydrogen storage development.
In June, Japan and Australia announced a possible submarine development deal as they stepped up their defence ties.
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