Japan's government is preparing to make a decision in late August on when to start releasing treated and diluted water from the crippled Fukushima Daiichi nuclear power plant into the ocean.

The government is making arrangements to hold a meeting of relevant Cabinet ministers to make a decision after Prime Minister Kishida Fumio's visit to the United States.

Officials are expected to consider starting the release of the treated water sometime between late August and the first half of September.

Kishida is scheduled to visit the US from Thursday next week for a trilateral summit with President Joe Biden and South Korean President Yoon Suk-yeol.

Government sources say the prime minister is expected to hold separate talks with the two leaders to explain the water discharge plan and seek their understanding.

Rain and groundwater mixes with water used to cool molten fuel at the Fukushima Daiichi plant. The accumulated water is treated to remove most radioactive substances, but still contains tritium.

The Japanese government plans to dilute the treated water to reduce tritium levels to about one-seventh of the World Health Organization's guidelines for drinking water quality before releasing it into the sea.

Some government officials say the water release should start this month, as the trawl fishing season will open in waters off Fukushima Prefecture in September.

Last month, the International Atomic Energy Agency released a report saying Japan's water release plan is consistent with international safety standards.

But Japanese fishers and people in countries near Japan, such as China, are voicing opposition or concerns about the plan.

Chief Cabinet Secretary Matsuno Hirokazu told reporters on Monday that no decision has been made on the exact timing and details of the water release.

He said the government has been thoroughly explaining to local fishers how the safety of the planned move will be ensured.

Matsuno said government officials will keep working to provide explanations. He said they will also work hard to ensure safety, prevent reputational damage and protect the fishers' livelihoods.