As many as 140 million people in Bangladesh may be at risk from a huge earthquake as pressure builds beneath the surface of one of the world's most densely populated nations, US and Bangladeshi scientists say.

Sediment flows from the Ganges and Brahmaputra rivers have layered parts of the country with as much as 20 kilometres of sand and mud, masking until now the extension of the same fault line that triggered the 2004 Sumatra tsunami and killed 230,000 people.

"The fault is entirely in the sub-surface," said Michael Steckler, a geophysicist at Columbia University's Lamont-Doherty Earth Observatory and lead author of a paper published this week in Nature Geoscience. "It's been suspected but we haven't had the data to prove that [the pressure] is actually been building up."

Data collected since 2004 by Professor Steckler's team has found that a juncture between major tectonic plates in the region is locked and loading up with stress.