Shortly after Saturday's match kicked off in the Stade Roi Baudouin, heavy rain began to fall as Belgium pressed for an early opener.
Such a goal was not forthcoming, and midway through the half severe hail began to fall.
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Eventually the storm became too strong, with Kassai taking the players from the field in the 23rd minute, fearing for their safety as stones the size of golf balls beginning to bounce off the turf.
The players returned to the pitch and action resumed after a delay of more than 45 minutes.
Belgium will have been grateful for the referee's caution as the team flies out to Brazil to begin its World Cup campaign following Saturday's game, with a clash against Algeria coming up on June 17.