A boat carrying about 70 people has sunk on a major river in northern Brazil, leaving at least 10 dead and dozens more missing, according to authorities.

The bodies of a teenager and a child were among those recovered on Wednesday, state security officials said.

In an updated toll, the public security office of the state of Para said that only 15 people made it to the shore, not the 25 initially reported. At least 45 others were missing.

Authorities said the boat was travelling on the Xingu River when it sank late on Tuesday.

The cause was not immediately clear.

"Searches are continuing above and below water," Augusto Lima, from the Para firefighters' service, told the Folha de S. Paulo newspaper.

"It's a hard-to-access area, and of the three aircraft we sent, one is there just to act as a communications base," he said.

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The Folha newspaper reported that the vessel left on Monday night from Santarem and was heading to Vitoria do Xingu.

Travel by river is common in Brazil's northern states, which include the Amazon rainforest and have relatively few major roads.

In early August, a cargo vessel collided with a tugboat on the Amazon River, also in the state of Para.

Only two people were rescued out of 11 aboard the tugboat.