"A bomb blast in front of the Laghman General Prison left three civilians killed and four others, including prison director Noor Mohammad Niazi, injured", Asadullah Dawlatzai said, adding that it was a motorcycle bomb.
Locals revealed that the explosion took place at 11 a.m. local time (06:30 GMT).
No militant group has claimed responsibility for the blast yet.
Earlier this week, four Afghan policemen were killed and five others were injured as Taliban militants carried out an attack on a security checkpoint in the Nachin area of the country's central province of Oruzgan.