"These are very exceptional circumstances, without any precedent in our country," he said.
Interior Minister Annelies Verlinden said the death toll from the floods had risen to 20 people, with around 20 people missing. Earlier media reports had said 23 dead.
More than 120 people have died so far in floods that have devastated large areas of Germany, Belgium, Luxembourg and the Netherlands.
In Belgium, waters have begun to recede and torrential rains have ceased, but thousands of emergency responders are still at work.
De Croo said flags on Tuesday will be flown at half-mast and a minute of silence would be held at midday.
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