China is playing by different rules in its territorial negotiations with Japan concerning disputed areas of the East China Sea that are believed to hold deposits of natural gas and oil, according to sources in Japan's Foreign Ministry.
While it was in China's best interests to sign an agreement with Vietnam that recognised a median line between the two countries as their sea border, Beijing is instead insisting on using the continental shelf in an attempt to extend its claims to more of the ocean - and what lies beneath its seabed - in discussions with Tokyo, according to the officials, who are involved in the talks.
'The agreement with Vietnam used the phrase 'equitable principles', which seemed to be a favourite [term] of the Chinese, and the line was drawn almost exactly on the median line because that was the best for them in that situation,' a Japanese Foreign Ministry official said. 'But they are not using that argument with us.'
Citing previous cases in which disagreements have been decided by the International Court of Justice, Japanese officials said Tokyo would be happy to have independent arbitration in the matter - but China had refused to consider the suggestion.
'We have been saying since 1968 that we would abide by the ICJ's decision in any territorial dispute, but China refuses to go to a tribunal and it cannot be forced to go,' the Japanese official said.
'In similar cases, the court has ruled that it cannot make a decision because one country is not represented.
'China is not playing by the rules. It is not an issue of business or money for Japan. It's an issue of sovereignty, and the Chinese side's claims are totally unfounded.'
The two sides have held three rounds of talks on the matter since last October. The most recent was last Saturday, during which Tokyo proposed joint development of oil and gas reserves in fields that straddle the median line.
Japan is awaiting a response from Beijing on the proposal and is hopeful that the date for a new round of talks can be agreed, possibly within the month.
'The first two rounds of talks were very tough, but at the director-general level they are getting slightly better now,' the Japanese diplomat said.
'Between legal experts, there is still a very hard atmosphere, with the Chinese side simply reiterating the same position. We hope to receive a reply on joint development at the next round.
'Are they going to say 'yes'? Probably not,' he admitted. 'But at the same time, we simply cannot accept their proposals.'