Basing Japan's missile defense systems at sea may cost at least twice as much to complete as its now-abandoned plans for Aegis Ashore ground-based sites and delay the rollout to 2028, a person with knowledge of the plans has said.

Fitted with powerful Lockheed Martin Corp. radars, Japan's Aegis Ashore systems were meant to intercept missile strikes from North Korea and elsewhere. In June, then-Defense Minister Taro Kono suspended plans for two land sites, which would have cost about $2 billion to construct, citing the possibility that booster rockets could fall on local residents.

Instead, he suggested installing the systems on sea platforms or ships.