Nobody was hurt in the blaze at the third floor of the complex at the Parliament street.
"The fire broke out at room number 333 on the third floor. Prima facie, it appears that the blaze erupted in a split air conditioner in the room.
"The exact reason behind the fire though will be ascertained only after police probe in the incident," a fire official said, adding, the fire spread to other areas.
He said six fire tenders were rushed to the building to douse the fire on a room of the Transport Ministry after receiving a call at around 3:50 PM.
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Asked whether some files got damaged in the blaze, both Fire department and Police did not give a specific answer.
An employee of Transport department said "We heard sound of blast after which smoke engulfed the room leading to darkness. Some of us started suffocating and immediately rushed out. Luckily none was hurt."
The employees were allowed to enter the premise at around 5 PM after situation was brought under control.