An ageing US ‘nuclear coffin’ on an uninhabited Pacific island is an ecological disaster waiting to happen, the head of the United Nations has warned.

The concrete ‘lid’ was built on Enewetak Atoll in the Marshall Islands to contain radioactive material from American nuclear tests in the 1950s.

But rising sea levels caused by climate change could damage the "kind of coffin", UN Secretary-General Antonio Guterres warned on a visit to Fiji earlier this week, reports CBS.

The concrete ‘lid’ was built on Enewetak Atoll in the Marshall Islands to hold nuclear materials. (Supplied)

A file photo showing the damage on a Pacific island from a nuclear bomb test in the 1950s. (Getty)

He said the 45-centimetre thick dome is vulnerable, and that a strong storm could release the radioactive materials into the Pacific Ocean.

Mr Guterres said Marshall Islands President Hilda Heine told him she is "very worried" about the possible leak.

From 1946 to 1958, the US staged 67 nuclear tests in the Marshall Islands during the Cold War. As well as the Pacific, US and British nuclear weapons were also held in other remote parts of the world including South Australia as the west strived to match Soviet Russia’s nuclear build up.

The remote islands and atolls were blasted by nuclear and thermonuclear bombs, including the notorious Castle Bravo test.

From 1946 to 1958, the US staged 67 nuclear tests in the Marshall Islands. (Getty)

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The 10-tonne device released an unanticipated explosive yield of 15,000 kilotons of TNT. In comparison, the yield of the atom bomb that wiped out the Japanese city of Hiroshima in 1945 had jut 15 kilotons.

To deal with the fallout from the Castle Bravo explosion and other nuclear tests, the US military decided to fill a 100-metre wide crater on the island of Runit.

US servicemen and civilian contractors dumped radioactive sand, soil and other materials into it.

A file photo showing damage on the Marshall Islands caused by high seas. UN Secretary-General Antonio Guterres has warned climate change threatens the nuclear storage dome there. (Getty)

In 1980, a concrete dome was built over the crater and sealed the radioactive waste inside.

Despite its vast size – enabling it to be observed from space – the dome is showing signs of cracking.

Seawater has also penetrated it through the crater’s unlined interior.

Since 1979, the US ceded responsibility for the dome to the government of the Marshall Islands. But the tiny, cash strapped Pacific nation doesn’t have the resources to move the radioactive material to a new home.

Mr Guterres visit to Fiji was part of a trip to Pacific island nations to see the effects of climate change.