"We have conveyed to the Chinese side our country's concerns that if this is realized, it could violate our sovereign rights and provisions in the U.N. Law of the Sea," Chief Cabinet Secretary Shinzo Abe told a news conference Monday, Japan's Kyodo news agency reported.
Prime Minister Junichiro Koizumi called for a level-headed approach to the issue, saying: "I wonder what this means. We would like to deal with it in a level-headed manner."
Chinese maritime authorities served notice in March, banning ship traffic around disputed gas fields in the East China Sea while expansion operations were under way at the state-owned Pinghu gas field.
Japan has asked China to confirm whether the expansion work is planned or has already started.
China has promised details through diplomatic channels as quickly as possible, Kyodo said.