Authorities in Bulgaria were on high alert after a bomb was reportedly discovered an explosive device inside a minibus parked outside Sofia Airport Terminal 1.

Passengers and staff have been evacuated from Terminal 1. Flights will go on schedule according to airport officials, but takeoff and landing are both redirected to adjacent Terminal 2, with authorities promising to mitigate delays.

Earlier media reports suggested a "suspicious package" had been found inside a white minibus with Belgian license plates.

Police have not confirmed an explosive device or substance were found, Nova TV said. The Interior Ministry's press office denied information from an airport official who was heard telling several national TV stations a device had been discovered.

A search is still underway.

The area around Sofia Airport Terminal 1 has been sealed off.

Reportedly, the minivan was parked by a non-Bulgarian national around 01:00 local time (EET).

His flight took off around 05:00 leaving the vehicle in front of Terminal 1, police say, without elaborating.

Police carried out a controlled explosion of one of the bags found inside the minivan.

Officials at the Interior Ministry explained that a police dog trained to discover explosive devices had reacted "positively" during the first routine check of the minivan on Tuesday afternoon.

However, that reaction was not tantamount to "a bomb inside", the Bulgarian National Radio quotes them as saying.