LONDON - A major search is under way after a teenage girl was stabbed to death on a bus on her way to school in Birmingham, police said Thursday.

The 16-year-old was on a crowded double decker bus when the attack happened at around 7.30am during morning rush hour, close to the city centre.

West Midlands Police warned the public to be "vigilant" as officers from across the force searched for a black man in his late teens or early 20s.

Witness said he was wearing dark coloured clothes, including a jacket with a tiger's head or other large motif on the back.

Detective Superintendent Richard Baker urged members of the public to report any sightings of the man and not to approach him as he may still be in possession of weapons.

He said: "This is a tragic case and the victim's family are devastated. Few of us can imagine what they are going through and we are working hard to identify the offender which involves speaking to a large number of people who witnessed the attack.

"From their descriptions, we are keen to identify a young black man in dark clothing who was on the bus and seen fleeing from the area around the time of the murder.

"I would urge the public not to approach the man but to immediately dial 999. We have officers ready to respond immediately to sightings".

Man detained, but released

The force said dog handlers, firearms officers and transport police were involved in the search, while forensic officers were scouring the scene where it is hoped CCTV cameras may have captured the incident.

A man was detained by police in connection with the stabbing but later released without arrest.

Police were alerted to the incident on Hagley Road by the driver of the bus, who reported that a passenger had been wounded.

"Officers and other emergency services were immediately dispatched but despite valiant efforts to save the teen, it was clear that this was a fatal attack," a police spokesman said.