BRASÍLIA (AFP) - Sexual assaults in Brazil dipped according to statistics released on Friday (Oct 9), but researchers said the apparent decline only masked a huge national problem.

The Brazilian Forum on Public Security, a respected research group, said there were 47,643 registered sex assault cases in 2014, down from 51,090 the previous year.

However, "this crime has the highest rates of non-reporting," said the group's director Samira Bueno, "so it is hard to evaluate whether there was truly any reduction".

Even at the lower number, an assault takes place every 11 minutes in the country, the group said, while according to a government study barely 10 per cent of all cases are reported to the police.

In the northern state of Roraima, on the border of Venezuela, the number of such cases reached a rate of 55.5 for every 100,000 people, more than double the national average of 23.5, the study found.

Rio de Janeiro, which hosts next year's Olympics, saw a rate of 34.5.

A poll by Datafolha found that 67 per cent of Brazilians fear a sexual assault.