Japan plans to buy at least 20 additional F-35A stealth fighters over the next six years, some or all of which it may purchase directly from U.S.-based Lockheed Martin Corp. rather than having them assembled by Mitsubishi Heavy Industries Ltd., its primary defense contractor, three sources said.

"In view of budgets and production schedules a new acquisition of around 25 planes is appropriate," said one of the sources with knowledge of the plan. The sources asked not to be identified because they are not authorized to speak to the media.

The sources said buying complete aircraft from the U.S., at about $100 million each, will save Japan about $30 million per airframe.