Japan plans to develop unmanned underwater vehicles to lay and sweep mines, aiming to attain maritime superiority in the event of a contingency near the country's southwestern Nansei Islands chain, a source close to the matter said Monday.

The plan comes as the Defense Ministry seeks to ramp up the use of drones to support missions of the ground, maritime and air branches of the Self-Defense Forces.

It is expected to be articulated in the National Security Strategy and two other key defense documents slated to be revised by the end of this year.