(Adds media report on timing of decision in paragraphs 8-9)

TOKYO, Aug 19 (Reuters) - Japanese Prime Minister Fumio Kishida said he will visit the Fukushima nuclear plant on Sunday ahead of a final decision on when to begin releasing wastewater from the wrecked facility into the Pacific Ocean.

"The government is at the final stage of when it has to make a decision," Kishida said on Friday in the U.S. following a trilateral meeting with South Korean President Yoon Suk Yeol and President Joe Biden.

He declined to say when the water release will begin during the briefing, which was aired by public broadcaster NHK.

Japan plans to release into the ocean 500 Olympic-size swimming pools worth of water from the plant wrecked by a tsunami on March 11, 2011.

The International Atomic Energy Agency (IAEA) last month greenlit the release of radioactive water that Japan says it can no longer store on site, but Tokyo held off on doing so ahead of the meeting in Washington to avoid stirring political opposition in South Korea to Yoon, sources earlier told Reuters

On Friday the three countries agreed to deepen military ties to counter China's growing influence in East Asia.

The plan to pump more than a million tons of treated water into the Pacific from the nuclear plant owned by Tokyo Electric Power Company (Tepco) has been criticized by Beijing, which has banned some seafood imports from Japan. It is also opposed by some citizens' groups in Japan, South Korea and elsewhere.

Kishida will meet with senior members of organisations representing fishermen on Monday to assert the safety of the treated water and seek their understanding, the Asahi newspaper reported citing government sources.

The government will make a final decision on the timing of the water release at a meeting of ministers as soon as the following day, Asahi said.

Japan said it will remove most radioactive elements from the water except for tritium, a hydrogen isotope that must be diluted because it is difficult to filter. (Reporting by Tim Kelly; Editing by Sonali Paul and Sandra Maler)