The blast happened late Friday at the 10-storey Kawish Crown Plaza building near Karachi's Quaid-e-Azam international airport at about 1530 GMT, AFP reported
There were no reported casualties.
“The floor was empty at the time of explosion," said Tariq Jameel, a senior police official.
So far no one has claimed responsibility for the blast though a police official said it may be linked to local business rivalries.
Karachi is reknowned for its violent crime. Since the US-led war against Afghanistan the port city has witnessed an exponential increase in gun attacks and Kalashnikov assault rifles can reportedly be hired on an hour by hour basis.
Political instability and the distraction of sparring with nuclear-rival India has allowed criminal gangs to increase their influence.
Further, Karachi has been the scene of several bloody attacks mainly on western and Christian targets in recent years.
Islamist opposition groups angered by Pakistan's support for the Afghan war and the continued hunt for al-Qaida suspects have been blamed for these attacks.
In May 2002, a bomber exploded himself in his vehicle, killing 11 French navy officials and three Pakistani nationals outside the Sheraton Hotel.
In June last year, a car bomb outside the US consulate killed at least 11 people and wounded a further 45.
Political instability and the distraction of sparring with nuclear rival India has allowed criminal gangs to increase their influence.
Armed robbery in the city rose by 7.4% in the first half of this year. Some 1,150 aggravated incidents were reported.