Image copyright AP Image caption Rescue officials at the scene of an early morning collision between a school bus and a commuter bus in Baltimore

A school bus has crashed into a city transport bus in Baltimore, killing six and injuring 10 others, say police.

The yellow school bus hit a car before colliding into the side of the city bus that was carrying early morning commuters.

The accident killed the school bus driver and five others on the city bus. There were no school children on board.

Photos show the school bus' front end crumpled into the commuter bus' flank. The cause of the crash was unclear.

Image copyright AP Image caption A police officer walks past a car that was damaged by a school bus before the fatal collision

"It literally looks like a bomb exploded in the bus and it's catastrophic damage," said Baltimore police spokesman TJ Smith.

He told a news conference the school bus rear-ended a Ford Mustang before hitting a pillar at a cemetery and veering into oncoming traffic, colliding with the Maryland Transit Administration bus on the driver's side.

Ten people were taken to area hospitals for injures which ranged "from minor to critical", he added.

Those included eight people on the city bus, and an aide who was the only passenger on the school bus.

Image copyright WBAL Image caption An aerial view shows the carnage caused by the collision

Police shut down traffic in the area of southwest Baltimore where the crash occurred.

The severely damaged Ford Mustang was found over a block away near a large skid mark trail.

Police said the medical examiner would determine whether a health emergency led to the accident.

"There aren't any skid marks. So something catastrophic took place and it's something we're continuing to investigate," said Mr Smith.

The bus had been on its way to pick up a student a few blocks away.