Japan's national security adviser has told his US counterpart that Tokyo is studying alternatives to the US-designed Aegis Ashore missile defense system.

Kitamura Shigeru met Robert O'Brien in Washington on Thursday.

Japan's Defense Ministry says it is considering sea-based alternatives to the Aegis Ashore missile defense system. Earlier this year, the ministry abandoned a plan to deploy the land-based system at two locations.

Defense Minister Kishi Nobuo disclosed the sea-based options at a meeting of ruling Liberal Democratic Party lawmakers on Thursday.