OKUMA, Japan (AP) — The tsunami-wrecked Fukushima Daiichi nuclear power plant began releasing its first batch of treated radioactive water into the Pacific Ocean on Thursday — a controversial step that prompted China to ban seafood from Japan.

People inside and outside the country protested the wastewater release, with Japanese fishing groups fearing it will further damage the reputation of their seafood and groups in China and South Korea raising concerns, making it a political and diplomatic issue.

In response to the wastewater release, Chinese customs authorities banned seafood from Japan, customs authorities announced Thursday. The ban started immediately and will affect all imports of “aquatic products” including seafood, according to the notice. Authorities said they will “dynamically adjust relevant regulatory measures as appropriate to prevent the risks of nuclear-contaminated water discharge to the health and food safety of our country.”

Shortly after China’s announcement, Tokyo Electric Power Company Holdings President Tomoaki Kobayakawa said the utility was preparing to compensate Japanese business owners appropriately for damages suffered by export bans from “the foreign government” over the wastewater release. He said China is Japan’s key trading partner and that he will do his utmost by providing scientific explanations of the release so that the ban will be dropped as soon as possible.

The Japanese government and TEPCO say the water must be released to make room for the plant’s decommissioning and to prevent accidental leaks. They say the treatment and dilution will make the wastewater safer than international standards and its environmental impact will be negligibly small.

Tony Hooker, director of the Center for Radiation Research, Education, Innovation at the University of Adelaide, said the water released from the Fukushima plant is safe. “It certainly is well below the World Health Organization drinking water guidelines,” he said. “It’s safe.”

The water release begins more than 12 years after the March 2011 nuclear meltdowns, caused by a massive earthquake and tsunami. It marks a milestone for the plant’s battle with an ever-growing radioactive water stockpile that TEPCO and the government say has hampered the daunting task of removing the fatally toxic melted debris from the reactors.

The pump activated Thursday afternoon sent the first batch of the diluted, treated water from a mixing pool to a secondary pool 10 minutes later. It then moves through a connected undersea tunnel to go out 1 kilometer (0.6 miles) off the coast. Officials said the water moves at a walking speed and will take about 30 minutes to exit from the tunnel.