As U.S. climate envoy John Kerry appeared in Seoul over the weekend to discuss global warming,
South Korea's foreign ministry says it raised concerns to him over Japan's plans to dump contaminated water from its defunct Fukushima nuclear plant into the sea.
On Sunday however, Kerry reaffirmed Washington's confidence in the plan's transparency.
He said he believed Tokyo was working closely with the International Atomic Energy Agency, theorganisation that promotes the peaceful use of nuclear power.
"The United States is confident that the government of Japan has had very full consultations with the IAEA, that the IAEA has set up a very rigorous process and I know that Japan has weighed all the options and the effects and they've been very transparent about the decision and the process."
Under the plan, more than 1 million tonnes of water will be discharged from the plant, the site of the world's worst nuclear disaster since Chernobyl.
Seoul has strongly rebuked the decision.
The former U.S. secretary of state said Washington would closely monitor Japan's plan, to make sure there is no public health threat.
His visit to Seoul precedes U.S. President Joe Biden's virtual summit with world leaders on climate change, set for two days starting April 22.