Japan's ambassador to China warned last month that plans by the Tokyo municipal government to buy islands in the East China Sea claimed by Beijing but administered by Japan could trigger an "extremely grave crisis" between East Asia's two top powers.

Uichiro Niwa, a former chairman of the Itochu trading conglomerate who in 2010 became the first Japanese ambassador to Beijing appointed from the private sector, told Britain's Financial Times that the move would damage key economic ties between China and Japan, which has been overtaken by the former as the world's second largest economy.

In a provocative move, Tokyo Gov. Shintaro Ishihara, a high-profile Japanese nationalist, launched an appeal in April for money to purchase islands in the disputed Senkaku group, which China calls the Diaoyu islands.