TOKYO — The Japanese government said Tuesday it had decided to release into the sea more than 1 million tons of water that has collected at the wrecked Fukushima nuclear power plant, despite opposition from local fishermen and nearby countries.

The combination of cooling water and groundwater seepage had become contaminated from contact with reactors that melted down during the 2011 nuclear disaster, and initial attempts to treat it were only partially successful, leaving significant levels of radioactive isotopes.

The government says the water will be treated further to remove dangerous isotopes and diluted to well below World Health Organization standards for drinking water, in an operation to be supervised by the International Atomic Energy Agency (IAEA).

Tokyo Electric Power Co., known as Tepco, has collected the water — equivalent to about 500 Olympic-size swimming pools — in more than 1,000 metal tanks at the Fukushima Daiichi nuclear power plant. But it says it is running out of room to build more tanks and will start releasing the water into the Pacific Ocean in two years, in a process that is expected to last two to three decades.

“Disposing of the treated water is an unavoidable issue in decommissioning the Fukushima Daiichi plant,” Prime Minister Yoshihide Suga told reporters Tuesday, adding that authorities will ensure safety standards are exceeded and that steps are taken to prevent reputational damage.

The plan to release the water has infuriated Fukushima’s fishing community, only now getting back on its feet after the nuclear disaster and subsequent contamination. The industry fears that even if the water is safe, its discharge into the ocean may undermine confidence in the region’s produce.

“Seeing this decision being made is completely outrageous and it is something we absolutely cannot accept. We will strongly protest,” Hiroshi Kishi, the president of JF Zengyoren, the nationwide federation of Japanese fisheries cooperatives, said in a statement.

Tepco President Tomoaki Kobayakawa told Japanese media the company would “do its utmost” to prevent reputational damage as a result of the discharge, and would compensate anyone affected.

The government and Tepco say there is a strong scientific reason to be confident that ocean discharge is safe.

The water has already been or will be cleaned with an advanced treatment system, known as ALPS, to remove almost all radionuclides, including strontium and cesium.

What would be left is tritium — a naturally occurring isotope of hydrogen that is far less dangerous to human health and is routinely released into the ocean by nuclear power plants — and traces of carbon-14, a radioactive isotope of carbon, that would be so small as not to pose a significant risk, the government says.

If all the treated water were released into the sea over one year — as opposed to decades — the radiation impact for local people would be no more than one-thousandth of the exposure impact of natural radiation in Japan, the government says.

 IAEA Director General Rafael Mariano Grossi welcomed the decision to release the water, saying the organization would work closely with Japan before, during and after the water is discharged. That will include monitoring the environmental impact, he said.

“Our cooperation and our presence will help build confidence in Japan and beyond that the water disposal is carried out without an adverse impact on human health and the environment,” he said.

The United States lent its support to Japan’s decision, but China and South Korea were unhappy.

“In this unique and challenging situation, Japan has weighed the options and effects, has been transparent about its decision, and appears to have adopted an approach in accordance with globally accepted nuclear safety standards,” the State Department said in a statement.

China’s Foreign Ministry called the decision “highly irresponsible,” fueling nationalist anger on social media about the move and the fact that it was backed by Washington.

South Korea, another rival of Japan, also expressed “grave concerns” about the safety of its people and the environmental impact. Both countries, however, also operate nuclear reactors that release tritium into the environment, through water or steam, in comparable quantities.

Greenpeace, which opposes nuclear power, also condemned the decision, which it said disregarded the rights and interests of the people in Fukushima, in Japan and in the Asia-Pacific region. It argues the carbon-14 in the water can become absorbed into organic matter and become concentrated in the food chain.

But radiation experts play down the concerns.

“There is no health risk associated with the release of this water,” said Gerry Thomas, chair of molecular pathology at Imperial College London, noting that even if ingested, both tritium and carbon-14 pass quickly through the body and would only impart a “vanishingly low” dose of radiation.

Japanese authorities already conduct extensive testing on food and fish from Fukushima prefecture to ensure it is safe, setting much stricter limits for the amount of cesium allowed than the United States and the European Union. It is rare for any sample to exceed those limits.

However, Japan’s government and Tepco have faced public mistrust for their handling of the Fukushima disaster and its aftermath. Both were slow to admit that three of the reactor cores had suffered meltdowns, and they have been criticized for playing down bad news.

For years, Tepco claimed that the treated water stored at the plant contained only tritium, but data on its website showed that the treatment process had failed to remove many dangerous radionuclides, partly because of the need to process huge quantities of water in a hurry after the accident.

Finally, in 2018, it acknowledged that 70 percent of the water is still contaminated with dangerous radioactive elements — including strontium-90, a radionuclide that can cause cancer — and will have to be treated again before release.

Julia Mio Inuma in Tokyo and Min Joo Kim and Lyric Li in Seoul contributed to this report.