Japan expressed opposition Wednesday to exploitation by China of a natural gas field that may stretch into the seabed under contested waters in the East China Sea, having received news Beijing is making moves to that end.

"We conveyed our serious concerns (to China) through diplomatic channels," Chief Cabinet Secretary Yoshihide Suga told a news conference.

In the latest development threatening to worsen bilateral relations that are already tense due to the dispute over the Senkaku islets in the sea, Japan has confirmed that a large Chinese crane ship is building a drilling platform at a point near the Japan-claimed border between their exclusive economic zones.