Solomon Islands Prime Minister Manasseh Sogavare has taken aim at the International Atomic Energy Agency's (IAEA) report on the Fukushima discharge.
The IAEA reviewed Japan's plan to release the treated nuclear wastewater into the ocean and found the plan, which is underway, met international safety standards.
Japan maintains it is safe and said it would not allow the discharge of water into the ocean which would endanger human health and the environment.
But at the UN General Assembly in New York, Sogavare said his country stands with like-minded Pacific Islanders and it is appalled at Japan's decision to go ahead with the release.
"The effect of this act is transboundary and intergenerational and is an attack on global trust and solidarity," he said.
He said the increased warming and acidification of the ocean over 30-years-plus poses worrying risk for his people's wellbeing and future.
"The message is clear: our lives, our people, do not matter."
Sogavare attacked the IAEA report, saying it was "inconclusive", and "the scientific data shared remains inadequate, incomplete and biased".
"These concerns were ignored. If this nuclear waste is safe, it should be stored in Japan.
"The fact that it is dumped into the sea shows that it is not safe."
Japan's state broadcaster reported earlier that day, IAEA's director-general Rafael Grossi expressed his agency's commitment to ensuring the safe release of treated and diluted water from the Fukushima plant.
Japan was given a right of reply in the UN chamber, and a representative of Japan - who was not named - said some of Sogavare's statement was not factually correct.
"The government of Japan has never allowed and will never allow the discharge of the water into the ocean which would endanger human health and the environment," the representative said.
She said the Pacific Islands Forum Foreign ministers' statement this month recognised the IAEA as the authority on nuclear safety.
She said it has been taking measures strictly abiding by relevant international law and being transparent.
"The government of Japan has always provided detailed explanation on this matter to the international community, including PIF members, based on scientific evidence in a transparent manner."