Two-thirds of the world's major deltas, home to nearly half a billion people, are caught in the scissors of sinking land and rising seas, according to a study published on Sunday.

The new findings, based on satellite images, show that 85 per cent of the 33 largest delta regions experienced severe flooding over the past decade, affecting 260,000 square kilometres.

Delta land vulnerable to serious flooding could expand by 50 per cent this century if ocean levels increase as expected under moderate climate change scenarios, the study projects.

Worst hit will be Asia, but heavily populated and farmed deltas on every continent except Australia and Antarctica are in peril, it says.

On a five-tier scale, three of the 11 deltas in the highest-risk category are in China: The Yellow River delta in the north, the Yangtze River delta near Shanghai, and the Pearl River Delta next to Guangzhou.

The Nile in Egypt, the Chao Phraya in Thailand and the Rhone River delta in France are also in the top tier of danger.

Just below these in vulnerability are seven other highly-populated deltas, including the Ganges in Bangladesh, the Irrawaddy in Myanmar (Burma), the Mekong in Vietnam and the Mississippi in the US.

These flood plains and others all face a double-barrelled threat, reports the study, published in the journal Nature Geoscience.

On the one side, a range of human activity - especially over the last half-century - has caused many delta regions to subside.

Without human interference, deltas naturally accumulate sediment as rivers swell and spread over vast areas of land.

But upstream damming and river diversions have held back the layers that would normally build up.

Intensive subsurface mining has also contributed mightily to the problem, notes the study, led by James Syvitski of the Institute of Arctic and Alpine Research at the University of Colorado.