Life in the oceans is at imminent risk of the worst spate of extinctions in millions of years due to threats such as climate change and over-fishing, a study shows.

Time was running short to counter hazards such as a collapse of coral reefs or a spread of low-oxygen "dead zones", according to the study led by the International Programme on the State of the Ocean (IPSO).

"We now face losing marine species and entire marine ecosystems, such as coral reefs, within a single generation," according to the study by 27 experts to be presented to the United Nations.

"Unless action is taken now, the consequences of our activities are at a high risk of causing, through the combined effects of climate change, over-exploitation, pollution and habitat loss, the next globally significant extinction event in the ocean," it said.

Scientists list five mass extinctions over 600 million years - most recently when the dinosaurs vanished 65 million years ago, apparently after an asteroid struck. Among others, the Permian period abruptly ended 250 million years ago.