BRASILIA • Brazil's intelligence agency said it was investigating all threats to next month's Rio Olympics after a presumed Brazilian Islamist group pledged allegiance to the Islamic State in Iraq and Syria (ISIS) militants less than three weeks before the Games.

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

The Brazilian authorities have stepped up security after 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 ISIS.

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

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

Since last Thursday's attack in Nice, where a truck ploughed through crowds during Bastille Day celebrations, Brazil has sought to reassure the global 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 sympathisers of radical groups.

They said their biggest concern 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 target the Olympics.

REUTERS