Japan on Thursday expressed opposition to China\'s move to unilaterally develop natural gas in the disputed East China Sea, amid tensions over a territorial dispute between Asia\'s two biggest energy users. \"Japan and China have agreed to make the East China Sea \'a sea of peace, friendship and cooperation.\' It is not acceptable if China is unilaterally developing natural gas resources there,\" Chief Cabinet Secretary Yoshihide Suga told a news conference, referring to media reports saying China\'s state-run oil companies plan to develop new gas fields near the disputed median line in the East China Sea. Suga also said the government is trying to verify the facts with the Chinese side. Citing Chinese industry officials, reports mentioned Wednesday that China National Offshore Oil Corporation (CNOOC) and several other firms plan to develop seven new gas fields in the area, adding that CNOOC is expected to submit the plan for government approval soon. Two of the seven gas fields are near China\'s median line with Japan, according to the report. Japan is worried that China may siphon gas from the Japanese side. Earlier this month, Japan confirmed a Chinese crane ship was building a mining facility about 26 kilometers into the Chinese side of the median line. Relations between the two countries have sharply deteriorated since Tokyo\'s nationalization of three of the five major Japan-administered islands in the East China Sea last September through purchase from a private Japanese owner. The small chain of uninhabited rocky islands, claimed by Japan, China and Taiwan, lie in rich fishing grounds and waters thought to contain large deposits of oil and natural gas. In 2008, Japan and China struck a landmark deal on joint development of natural gas fields in the East China Sea, but the talks on details have stalled.