The concrete dome was built in the late 1970s as a dumping ground for waste from the nuclear tests.

The United Nations Secretary-General, Antonio Guterres, has raised concerns that a concrete dome built last century to contain waste from atomic bomb tests is leaking radioactive material into the Pacific.

Speaking to students in Fiji on Thursday, Guterres described the structure on Enewetak Atoll in the Marshall Islands as “a kind of coffin” and said it was a legacy of Cold War-era nuclear tests in the Pacific.

“The Pacific was victimised in the past as we all know,” said Guterres, referring to nuclear explosions carried out by the United States and France in the region.

In the Marshalls, numerous islanders were forcibly evacuated from ancestral lands and resettled, while thousands more were exposed to radioactive fallout.

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The island nation was ground zero for 67 American nuclear weapons tests from 1946 to 1958 at Bikini and Enewetak atolls, when it was under US administration.

The tests included the 1954 “Bravo” hydrogen bomb, the most powerful detonated by the US, about 1,000 times bigger than the atomic bomb dropped on Hiroshima.

Guterres, who is touring the South Pacific to raise awareness of climate change issues, said Pacific islanders still needed help to deal with the fallout of the nuclear testing.

“The consequences of these have been quite dramatic, in relation to health, in relation to the poisoning of waters in some areas,” he said.

“I’ve just been with the president of the Marshall Islands [Hilda Heine], who is very worried because there is a risk of leaking of radioactive materials that are contained in a kind of coffin in the area.”

The “coffin” is a concrete dome, built in the late 1970s on Runit Island, part of Enewetak Atoll, as a dumping ground for waste from the nuclear tests.

Radioactive soil and ash from the explosions were tipped in a crater and capped with a concrete dome 45cm thick.

However, it was only envisaged as a temporary fix and the bottom of the crater was never lined, leading to fears the waste is leaching into the Pacific.

Cracks have also developed in the concrete after decades of exposure and there are concerns it could break apart if hit by a tropical cyclone.