China's biggest salt producer urged people against panic buying after Japan began discharging treated radioactive water into the Pacific Ocean from the wrecked Fukushima nuclear plant on Thursday, despite firm opposition from Beijing.

The state-run National Salt Industry Group, the world's biggest common salt producer, said in a statement issued hours later that it was ramping up supplies as people in some parts of the country rushed to stock up.

Supermarket salt shelves were emptied in many places like Beijing and Shanghai, and stocks also ran out on some online platforms.

China has opposed Japan's action, saying the Tokyo had not done enough to prove that the water discharged would be safe and has banned the import of all aquatic products from Japan.

"We are working overtime to produce, distribute and making all efforts to guarantee market supply," the National Salt Industry Group said in its statement.

"Please purchase rationally and do not panic buy blindly," it said.

The group said its salt supply is ample and the stock shortfall would be temporary. (Reuters)