U.S. President Joe Biden wants to lock in friendly ties between Japan and South Korea at a summit on Friday, but their readiness to shelve grievances will be tested when Tokyo begins pumping water from its wrecked Fukushima nuclear plant into the sea.

Japan already delayed the release to avoid stirring up political opposition in South Korea before President Yoon Suk-yeol joins Prime Minister Fumio Kishida for a meeting with Biden at the Camp David retreat on Friday, four officials in Japan and South Korea said.

The dumping of radioactive water may happen days after the summit, which the United States is billing as a "historical" trilateral meeting that will deliver a "bold counter" to regional rival China.