RIO DE JANEIRO (AFP) - An attack on a bus ferrying journalists at the Olympic Games was an "act of vandalism" carried out with rocks, the Rio 2016 security chief said Wednesday (Aug 10).

"Stones were thrown and hit the metal part of the windows of the bus," Luiz Fernando Correa told a press conference.

Correa said security along the route was being beefed up as a result of the attack, which witnesses on board the bus blamed on gunfire.

Three people on board suffered minor injuries as windows shattered after being hit.

Rio 2016 spokesman Mario Andrada said forensic analysis carried out on the bus had determined a rock was used in the attack.

"The first finding of the forensic study confirms the bus was hit by a rock which is also worrying and intolerable," Andrada said.

"The complete findings of the forensic team will be made available later today."

Correa defended security along the bus route and said it would be "impossible" to provide complete protection.

"It would be humanly impossible to exclude a person within range of throwing," Correa said.

"We think it's an act of vandalism rather than a criminal act with the intention of injuring someone."