"Thermal stress levels set new record highs in CNMI and the NWHI and we saw the first widespread bleaching event in the main Hawaiian Islands."

Mr Fellenius said coral bleaching was a naturally occurring phenomenon but not on the scale currently being seen.

Coral bleaching occurs when the photosynthesising algae on which the reef-building creatures depend for energy disappear. Deprived of these for even a few weeks, the corals die.

"While bleaching can occur on very hot days in pools of water with little circulation [such as] very low tides on reef flats, it has become a global problem due to greenhouse gas emissions causing elevated temperatures under climate change."

He said sea surface temperatures had been on average half to a full degree Celsius higher than normal for months, adding: "This does not seem like a lot but it makes a big difference to corals."

Mr Fellenius said the last major bleaching event was in 1997, when an exceptionally strong El Nino system affected about a quarter of the world's coral reefs.

He said indications were that the latest episode had affected up to 75 per cent of smaller corals and 25 per cent of the larger varieties at some sites in the Marshalls.

He said the bleached coral was becoming covered with algae, hindering its chances of recovery.

The World Meteorological Organisation (WMO) raised the alarm about rising sea temperatures this month on the sidelines of UN climate talks in Lima, saying 2014 was set to be the hottest year on record, consistent with man-made climate change.

"What is particularly unusual and alarming this year are the high temperatures of vast areas of the ocean surface," WMO chief Michel Jarraud said.

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Global warming blamed for 'worst ever' Marshall Islands coral bleaching