WASHINGTON (AFP) - At least six people were killed when a school bus, that was carrying no children at the time, collided with a commuter bus in the US city of Baltimore on Tuesday (Nov 1), police said.

Photos posted online showed the yellow school bus with its front end crumpled into the middle of the commuter bus.

First responders could be seen on the scene, which was strewn with debris.

"There were no children on the school bus, however, there are six fatalities," the Baltimore Police Department posted on Twitter.

Baltimore chief media relations officer T.J. Smith tweeted that authorities "are in the process of gathering information on the bus crash. We will advise once we have more. Understand this is a fluid situation."

The accident occurred at approximately 6.30am (1030 GMT) in the eastern city located some 25 kilometres north of Washington.