"It is extremely regrettable that [China] has unilaterally proceeded with the development while the boundary between Japan and China has not yet been fixed," Suga said, as quoted by Kyodo news agency.
In 2008, the two countries reached an agreement on joint gas production in the East China Sea, but in 2010 China pulled out of the deal after a Chinese fishing boat collided with a Japanese surveillance vessel in the region near the disputed area of the Senkaku Islands.
Tokyo said that Beijing had constructed 16 gas development facilities on locations too close to the median line dividing the Chinese and Japanese coastlines in the East China Sea. Earlier, the Japanese side said that the number of active gas production facilities reached seven.
The Senkaku Islands, known as the Diaoyu Islands in China, are the subject of long-standing regional territorial disputes between China, Japan and Taiwan. They lie on a vital shipping route and are situated above large hydrocarbon deposits.
