Japan has lodged a protest with Beijing after confirming that China installed drilling equipment in a prospective gas field in the East China Sea.
This is part of the area that Tokyo and Beijing agreed to jointly develop as gas fields in 2008.
A treaty on the joint development has yet to be signed, but China has been pushing ahead with development on its own, including setting up structures in the waters near an intermediate line between the countries.
In May, Tokyo confirmed that Chinese workers had set up the 17th structure of its kind in the region. Japan's Maritime Self-Defense Force has recently confirmed the installation of drilling equipment at that site, according to an announcement by the Foreign Ministry on Monday.
Japanese officials say they are also aware that, as of last Friday, work to transport the foundation of an 18th structure into the area was underway.
Funakoshi Takehiro, who heads the Foreign Ministry's Asian and Oceanian Affairs Bureau, called China's unilateral development "extremely regrettable."
Funakoshi urged Beijing to respect the 2008 agreement and return to the negotiating table to work out a joint development treaty.