The raid saw the gang of thieves hit a Brinks transport truck as its contents were being loaded onto a Swiss aircraft bound for Zurich, it said.
Belgian prosecutors confirmed that a raid had been carried out at the airport, but did not specify what was stolen. They were expected to provide more details later in the day.
A statement from the airport said the robbers drove through barriers on the tarmac to get airside.
Reports said no shots were fired, no-one was injured and the heist was completed within a matter of minutes.
One of the vehicles was found afterwards burnt out near the airport, local police added.