Japan conveyed "strong concern" to China on Thursday over its suspected gas field explorations in a contested area of the East China Sea as their senior diplomats held a teleconference, the Japanese government said.

But they also agreed to keep communicating toward the implementation of the 2008 bilateral agreement on joint gas development in the area, the Foreign Ministry said, although the negotiations have been stalled for more than a decade.

Takehiro Funakoshi, director general of the ministry's Asian and Oceanian Affairs Bureau, complained to Hong Liang, head of the Chinese Foreign Ministry's Department of Boundary and Ocean Affairs, about China's "unilateral resource development" in the sea, according to the Japanese ministry.

The first director general-level talks since last November were held after Japan said Monday it had confirmed that China set up a new drilling facility for gas fields on the west side of a Tokyo-proposed median line separating the two nations' exclusive economic zones in the sea.

File photo taken in September 2012 shows the Senkaku Islands in the East China Sea. (Kyodo)

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Last week, Japan also confirmed new construction work by China that is suspected to be for gas exploration at another spot on the Chinese side of the proposed median line. Japan has been aware of 18 such Chinese structures in the area including those under construction, the ministry said.

Talks to implement the 2008 agreement have been suspended since 2010 when tensions increased following a Chinese trawler's collision with a Japan Coast Guard vessel.

Japan regards the median line as the demarcation line between the two neighbors under domestic law, while Beijing says its EEZ extends much further.

Japan fears China's unilateral development in the area may lead to the siphoning off of resources from beneath the Japanese side of the line.

During the virtual meeting, Funakoshi also raised the issue of repeated intrusions by Chinese ships into Japanese waters around the Senkaku Islands in the sea, the ministry said. China claims the Japan-controlled, uninhabited islands.

Funakoshi "strongly demanded" that China deal with bilateral issues, also including its "intensifying military activities" such as a joint overflight by Chinese and Russian strategic bombers in May over the Sea of Japan, as well as a maritime survey conducted by a Chinese research vessel in Japan's EEZ early this month, the ministry said.

On the plan of establishing an emergency hotline between Japan's Self-Defense Forces and the Chinese People's Liberation Army to avoid unintentional contingencies, the two officials agreed to aim for its early creation.