Last week, China banned all seafood imports from its neighbour after Japan began releasing cooling water from the stricken Fukushima plant in an operation that Tokyo and the UN's nuclear watchdog have said is safe.

Since then, Japanese businesses ranging from bakeries to aquariums have received reportedly thousands of sometimes abusive crank calls from Chinese numbers.

"There have been numerous harassment calls believed to originate from China and instances of stones being thrown into the Japanese embassy and Japanese schools. It must be said these are regrettable," Fumio Kishida told reporters.

"We summoned the Chinese ambassador to Japan today and strongly urged him to call on Chinese people to act in a calm and responsible manner," Kishida added.

The comments came after Japan said it was heightening security at its diplomatic missions and schools in China.

Tokyo over the weekend told its tens of thousands of nationals living in China to keep a low profile and not to speak Japanese loudly in public.

Deputy Foreign Minister Masataka Okano told ambassador Wu Jianghao that China should properly inform the public "rather than unnecessarily raising people's concerns by providing information that is not based on scientific evidence", the foreign ministry said in a statement.

Social media users in China have posted recordings and videos of the nuisance calls, some of which have attracted tens of thousands of likes and a large number of comments.

"Since the start of the discharge... there have been numerous phone calls and other harassment regarding the discharge that are suspected of originating from China. The situation has not improved since then," Okano told Wu.

"A number of similar incidents are also happening in China against Japan-related facilities. This is extremely regrettable and we are deeply concerned," he said, according to the release.

Japan has begun releasing more than 500 Olympic swimming pools' worth of wastewater from Fukushima into the Pacific, 12 years after a tsunami knocked out three reactors in one of the world's worst atomic accidents.

Plant operator TEPCO says all radioactive elements have been filtered out except for tritium, levels of which are within safe limits.

Test results since the start of the discharge have confirmed this, according to Japanese authorities.