China has told Japan its blanket radiation testing on seafood imports from the neighboring country introduced last month is "a necessary measure" to protect its consumers, according to sources familiar with the bilateral relationship.

The testing is believed to have been adopted to pressure Tokyo over its plan to begin releasing treated radioactive water from the crippled Fukushima nuclear plant into the sea, possibly from late August.

Beijing has communicated through diplomatic channels its practice of detaining seafood imports from Japan at customs for up to a month before clearance. Meanwhile, Tokyo has expressed renewed concerns about this procedure, urging for "appropriate" customs processes, according to the sources.