Spokesman Benoit Ramacker told AFP that the previous toll of 31 included the three suicide bombers - two who blew themselves up at the airport and one on board a train stopped at Maalbeek station.
"The provisional toll is 28 people killed in the attacks," he said.
Only 24 of the victims have been identified to date, 14 of whom were killed at the airport and 10 at the train station.
The toll includes 13 Belgians and 11 foreigners from eight different countries, the Crisis Centre said.
A total 340 people from 19 countries were injured, of whom 101 remain in hospital. Sixty-two of them are in intensive care, half of them with severe burns.
"The toll, unfortunately, is still provisional due to the many people still in intensive care," the spokesman said.