China has put 16 drilling rigs close to its de facto maritime border with Japan, Tokyo said Wednesday, in the latest twist in a row over gas fields in waters disputed by the two countries.

The Japanese government released diagrams showing the location of platforms which it says could exploit undersea reserves over which the two countries are at loggerheads.

"It is extremely deplorable that China is unilaterally developing resources while the border has not been settled," Chief Cabinet Secretary Yoshihide Suga told reporters.

The platforms have all appeared despite a June 2008 agreement in which the two countries said they would jointly develop the area in the wake of friction over who had the rights to exploit the resources.

Of the 16, 12 have been installed in the last two years, Japan said.

The Japanese government's claims come a day after Tokyo slammed Beijing's bid to reclaim land in the South China Sea as a "coercive attempt" to force sweeping maritime claims in its annual defence paper.

The white paper accused Beijing of "raising concerns among the international community" in language that was markedly more stringent than in previous years.

China is locked in disputes with several countries over its claims to almost the entire South China Sea and is currently pursuing a rapid programme of artificial island construction in the region.

It is also embroiled in a separate row with Japan over the Tokyo-controlled Senkaku islands -- which it calls the Diaoyus -- in the East China Sea, as Chinese ships and aircraft regularly test Japanese forces in the area.

On Wednesday, Chief Cabinet Secretary Yoshihide Suga said: "Japan has repeatedly lodged protests against China's unilateral development."

"However, China has been negative on resuming talks over implementing the June 2008 agreement, even though its activities appear to be continuing."

Tokyo has long suspected that Beijing would violate the joint development agreement for the field, which lies in an area where both countries' claimed exclusive economic zones (EEZ) overlap.

Japan says the median line between the two nations should mark the limits of their respective EEZs.

But China insists the border should be drawn closer to Japan, taking into account the continental shelf and other features of the ocean.

The rigs are on China's side of the boundary, whichever measure is used.

In Beijing, the foreign ministry, responding Tuesday to Japan's white paper, defended its actions.

"China always carries out normal maritime activities following international laws and relevant domestic laws," a response posted on its website said.

"China's oil and gas exploration in undisputed waters of the East China Sea under China's jurisdiction is justified, reasonable and legitimate."