A powerful car bomb attack on a police bus in Pakistan's commercial hub of Karachi has killed at least 12 policemen and injured dozens more, officials said.

The blast went off early on Thursday morning as more than 50 officers were boarding the bus near the national highway in Karachi.

"Apparently an explosive-laden car hit the police bus transporting officials for security duty," Muhammad Iqbal, a senior police official, told the AFP news agency.

Doctor Semi Jamali at Karachi's Jinnah hospital told the AFP that in addition to the killed officers, about three dozen more policemen had been hospitalised, 10 of whom were in a critical condition.

The attack happened outside a police training centre, Iqbal said.

Police officer Rao Anwaar told the Associated Press news agency that it appeared to be a suicide attack, although there was no immediate claim of responsibility.

The port city of Karachi, Pakistan's business hub, has been plagued for years by sectarian, ethnic and political violence.

The assault is the latest in a series of attacks that come at a time when the Pakistani government is trying to strike a peace deal with local Taliban.

Source: Agencies