The death toll from Ecuador's biggest earthquake in decades has risen to at least 235 as rescue teams raced to find survivors in shattered coastal towns.

The powerful 7.8-magnitude quake struck off the Pacific coast on Saturday and was felt around the country, flattening buildings and buckling roads in several western towns.

Officials had previously put the toll at 77 dead and nearly 600 injured.

"The immediate priority is to rescue people in the rubble," President Rafael Correa, who was rushing home from a trip to Italy, said via Twitter.

More than 1,500 people were injured, authorities said.

The US Geological Survey said the shallow quake was centred 27km south-southeast of Muisne, a sparsely populated area of fishing ports that is popular with tourists.

Coastal areas nearest the quake were worst affected, especially Pedernales, a tourist spot with beaches and palm trees. Information was scant from there due to poor communications and transport chaos.

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