BRASILIA (Reuters) - Brazil's intelligence agency said on Tuesday it was investigating all threats to next month's Rio Olympics after a presumed Brazilian Islamist group pledged allegiance to Islamic State (IS) less than three weeks before the Games.

The SITE Intelligence Group that monitors the internet reported that a group calling itself "Ansar al-Khilafah Brazil" said on the Telegram messaging app on Sunday that it followed IS leader Abu Bakr al-Baghdadi and had promoted IS propaganda in Arabic, English and Portuguese.

Brazilian authorities stepped up security measures following the truck massacre in Nice last week, planning security cordons, further roadblocks and the frisking of visitors in Rio de Janeiro for the Olympics.

Police and soldiers took part over the weekend in drills near sports facilities and along transport routes.

The Games start on Aug. 5 and are expected to attract as many as 500,000 foreign visitors.

"All threats related to the Rio 2016 Games are being meticulously investigated, particularly those related to terrorism," the Brazilian intelligence agency ABIN said in a statement when asked to comment on the previously unknown group's claim of support for Islamic State.

"Many are dismissed and those that deserve attention are investigated exhaustively," ABIN said. An agency spokeswoman declined to comment on whether the posting by the group presented a credible threat.

ABIN last month confirmed it had detected a Portuguese account on the Telegram app that was a channel for exchanging information on Islamic State but authorities said no threat had been detected of an attack in Brazil.

Since Thursday's attack in Nice where a truck plowed through crowds during Bastille Day celebrations, Brazil has sought to reassure the international community that the Games will be safe and terrorist threats are being taken seriously.

On Monday, interim President Michel Temer issued a video message inviting foreigners to come to Rio and enjoy the Games and the beauty of the host city.

"We have reinforced security very much in the city and you can come without worries. You can enjoy the marvels of Rio de Janeiro and attend the Games," he said in the brief video.

Brazilian security officials say they are in close contact with partner countries about any possible threats to the Games and have been monitoring chatrooms and other communications among suspected sympathizers of radical groups.

They said their biggest concern during the Olympics is not the threat of a coordinated attack by known militants but the possibility that a lone actor or group sympathetic to militant causes could seek to target the event.

Brazil will deploy about 85,000 soldiers, police and other security personnel, more than twice the size of the security deployment during the London Olympics in 2012.

(Reporting by Anthony Boadle; Editing by Andrew Hay)