RIO DE JANEIRO (Reuters) - The U.S. Federal Bureau of Investigation gave Brazil the tip that led to arrests this week of 11 suspected militants who had discussed a possible attack on the upcoming Olympics in Rio de Janeiro, according to a Brazilian prosecutor.

In comments to several Brazilian newspapers published Saturday, Rafael Brum Miron, the federal prosecutor handling the case in the southern state of Parana, said the FBI had provided a brief report in which it identified at least six people it suspected as potential militants.

"The information came from the FBI," he told the Estado de S. Paulo newspaper. "They sent a succinct report: These people merit investigating."

A spokeswoman for the prosecutor's office confirmed the comments and said the FBI provided the tip in May, after which Brazilian investigators tracked the suspects' communications and identified the other people arrested this week.

A spokeswoman for the FBI in Washington declined to comment.

The arrests Thursday, just two weeks before the first-ever Olympics in South America begin Aug. 5, come after a recent wave of violent attacks in Europe and the United States and amid heightened fears that the Games could be a target.

Initially, police arrested 10 suspects and said that another two were being monitored. On Friday, one of the other two turned himself into police.

The suspects, described by Brazil's justice minister as poorly organized and "absolutely amateur," were alleged sympathizers of the Islamic State (IS) militant group.

Although some of the suspects had pledged allegiance to IS online, investigators said they have no formal ties or backing from the group itself.

Only two of the suspects actually knew one another, police said, but the group communicated extensively online and through messaging services.

In their messages, police said, the suspects applauded recent attacks outside Brazil and expressed desire - but little know-how and no specific plans - to stage an attack during the Olympics. Investigators said one of the suspects sought to buy an AK-47 rifle online from a vendor in neighboring Paraguay.

Despite inexperience or lack of organization, investigators say it is important to identify such groups and derail even incipient plans, especially at a time when IS and other militant groups are actively seeking to recruit sympathizers and would-be attackers.

"Are they amateurs?" the prosecutor asked in the Estado interview, before rhetorically answering his own question. "Yes, but I don't know of any experienced suicide bombers."

(Reporting by Paulo Prada in Rio de Janeiro and Roberta Rampton in Washington; Editing by Nick Zieminski)