Brasília (AFP) - Authorities in this year's Olympic host Brazil detected users exchanging messages in Portuguese linked to the Islamic State extremist group in an online web forum, intelligence services said Friday.

The warning raised security concerns about the potential existence of sympathizers of the armed group in Brazil, which was previously thought to be relatively free of Islamic extremism.

Pre-Olympic jitters had already risen after a French jihadist warned on Twitter after deadly attacks in France last November that Brazil was the "next target."

The Brazilian Intelligence Agency said in a statement Friday that it "confirms the existence of a group and its way of operating" online with jihadist messages.

It said the group was found on Telegram, an online messaging application.

"Content relating to extremist ideologies is translated to Portuguese and reproduced in that instant messaging application," the agency said.

The group was first detected by the SITE Intelligence Group, a non-government group that monitors extremist communications online.

SITE issued an alert saying that people linked to the Islamic State were thought to be spreading messages from the group in Portuguese.

Brazilian authorities are on high alert ahead of the Olympic Games in Rio de Janeiro from August 5 to 21.

They say more than 80,000 security personnel will police the Games. That is twice the number deployed at the last Olympics in London in 2012.

The Islamic State group has claimed responsibility for deadly attacks on civilians in various countries.

Authorities suspect many of its followers are radicalized and recruited online.

"The opening of this new front in the spreading of information for extremist indoctrination, aimed at the Portuguese-speaking public, increases the complexity of the job of tackling terrorism," the intelligence agency said in its statement.

"It represents an additional means of radicalizing Brazilian citizens."

Brazil is already reeling from recession and a political crisis.

President Dilma Rousseff has been suspended to face an impeachment trial.

The Senate is set to vote on whether to impeach her in mid-August, during the Games.