A huge underground river appears to be flowing thousands of metres beneath the Amazon River, Brazilian scientists said Thursday.

Valiya Hamza of Brazil's National Observatory said researchers found indications the subterranean river is 6,000 kilometres long, about the same length as the Amazon on the surface.

Hamza said the discovery of the possible underground river came from studying temperature variations at 241 inactive oil wells drilled in the 1970s and 1980s by Brazil's state-run oil company, Petrobras.

He said the "thermal information" provided by Petrobras allowed his team of researchers to identify the movement of water 4,000 metres under the Amazon River.

Their findings were presented last week in Rio de Janeiro at a meeting of the Brazilian Geophysical Society.

The apparent underground river has been named after Hamza, honouring him as the head of the research team that found the signs of the flowing water.

He said the existence of an underground river that also flows west to east would mean that the Amazon rainforest has two drainage systems — the Amazon and Hamza rivers.

Hamza stressed that the studies indicating the underground river were still in their preliminary stage but added that he expected to confirm the subterranean flow by the end of 2014.

He declined to comment on the economic and environmental impact of an underground river in the Amazon rain forest.