The United States and Japan plan to hold a meeting of coast guard commanders with Australia and the Philippines as part of their efforts to forge a unified front against China's maritime activities, according to diplomatic sources.

The four-way meeting of top coast guard officers, the first of its kind, is expected to discuss a future framework for working together to enhance maritime security, including cooperating in equipment procurement and holding joint drills, the sources said Monday.

The move came as the Defense Ministry announced that a Chinese naval intelligence vessel has been seen sailing outside the contiguous zone off the Boso Peninsula in Chiba Prefecture, several times since Thursday.