"We noticed an attempt to penetrate the Foreign Affairs network and decided, as a precaution, to isolate the network and proceeded to restart the system," he told reporters.
The network, called Diplobel, is used inside the country and by Belgian diplomatic missions abroad. Mr Lagatie said it will take time to restart.
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A message on the ministry’s website said all electronic services would be unavailable throughout the day due to an "urgent technical reason."
The servers will reportedly be back online within 72 hours.