Baku, August 20, AZERTAC An armed man is holding a number of passengers hostage on a commuter bus in the Brazilian city of Rio de Janeiro, according to BBC.Footage shot from a helicopter shows the bus stopped across two lanes on the bridge connecting Rio with the city of Niterói.Military police officers, ambulances and a special operations team are at the scene.At least six passengers have been freed. More than a dozen are still believed to be on board.Footage showed one of the released hostages fainting as she got off the bus. At one point the hostage-taker could be seen leaning out of the open door with something resembling a petrol bomb in his hand.Brazilian media say it is not clear if he has made any demands. He reportedly boarded the bus at 05:30 (0830 GMT) and threatened passengers.He reportedly told the passengers he was a military police officer. A spokesman for the force, Mauro Fliess, said it had not yet been confirmed if he was an officer or just claimed to be.One news site has released a WhatsApp message reportedly written by one of the passengers, saying that the gunman has tied the passengers' hands and has a petrol can as well as a gun.Traffic on the bridge has been blocked completely.Military police tweeted (in Portuguese) that the special operations team was negotiating with the hostage taker: "Our officers are trained and have great practical experience in this type of situations."