UPDATE, 4:32 p.m.: The motorcycle operator is listed in critical condition at a Boston hospital, where he was transferred after initially being taken to Baystate Medical Center. The man's name has not been released. An updated story will be posted.

SPRINGFIELD -- A man driving a motorcycle on State Street in Springfield sustained serious burns Tuesday afternoon following a collision with a sport utility vehicle at the intersection of Spring, School and State streets.

Witnesses at the scene said the motorcycle operator was driving away from the downtown area on State Street when he collided with a black Jeep Grand Cherokee that was making a left turn on State Street. A set of traffic lights controls the flow of vehicles at the intersection.

The motorcycle burst into flames upon impact, witnesses said.

Heavy damage to the Jeep's passenger-side front door was visible; the Jeep's rear bumper sustained fire damage, and the burned bike lay in the street as police investigated the crash.

Fire department spokesman Dennis G. Leger, aide to commissioner Joseph A. Conant, said Ladder 1 firefighters found the crash scene as they responded to a report of a gas leak at 315 State St.

The firefighters used two dry chemical fire extinguishers on the vehicles, and a pressurized water extinguisher on the motorcycle operator.

Patrolman Reggie Miller also happened across the crash scene and is credited with coming to the man's aid at risk to his own safety, Leger said.

Several bystanders, including 18-year-old city resident Terrell Diggs, said that prior to the firefighters' arrival they worked to pull the operator away from his motorcycle, which was still on fire. Diggs said he and others tried to extinguish the flames as first responders were en route.

The operator's helmet was on fire and much of his clothing was burned off, another witness said.

The condition of the motorcycle operator and the SUV occupants was not immediately known. A second motorcycle was seen on its side at the intersection of State and Myrtle streets, although it wasn't immediately clear if a second biker was involved or injured in the incident.

The eastbound lane of State Street at Spring and School streets, uphill to Federal Street, was closed as the investigation and cleanup continued.

This is a developing story. Additional information will be posted as it becomes available.