Already shaky Sino-Japanese relations are set to deteriorate further after China National Offshore Oil Corp (CNOOC) admitted that it had begun production in an East China Sea oil and gas field that is claimed by both nations. A spokesman for Japan's foreign ministry said Tokyo had yet to receive official confirmation of the reports, adding, "if indeed it is true, it is a source of concern", the Financial Times reported. Relations between the two countries have soured since Japanese Prime Minister Junichiro Koizumi came to office in 2001, largely because of his annual visits to the Yasukuni shrine, which honors several Class A war criminals as well as Japan's war dead. The dispute over the Chunxiao oil and gas field is long-running and despite several rounds of talks no resolution is in sight. It was reported Tuesday that CNOOC removed the original statement from its website announcing the start of production at Chunxiao.