China and Russia, which oppose Japan's planned release from Thursday of tritium-laced treated water from the crippled Fukushima No. 1 nuclear plant into the sea, jointly urged Tokyo last month to consider a vapor release disposal strategy instead, according to diplomatic sources.

Beijing and Moscow claimed in a document submitted to Tokyo in late July that vaporizing the water and releasing it into the atmosphere would have a smaller impact on neighboring countries compared with the ocean discharge plan. Japan has rejected their proposal, saying it is "impossible" to accept it, the sources said.

By Monday, the Japanese government had told China and Russia that it is difficult to conduct radiation monitoring in the atmosphere or predict how the vapor spreads, according to the sources.