DETROIT — Delta Air Lines canceled about 100 flights Sunday due to staff shortages, and it opened up middle seats a month earlier than expected in order to carry more passengers.

The airline says it had over 1 million passengers during the past few days, the highest number since before the coronavirus pandemic began last year.

“We apologize to our customers for the inconvenience, and the majority have been rebooked for the same travel day,” the airline said Sunday in a statement.

Delta took steps to increase passenger capacity, including opening middle seats on Sunday and Monday, in an effort to accommodate passengers.

On Wednesday, the airline announced that it would stop blocking off middle seats starting in May. The move was made last April to keep passengers farther apart, a policy that Delta’s CEO had repeatedly cited as raising trust in the airline. The seats would be reopened as air travel recovers and more people become vaccinated against COVID-19, the airline said.

Delta said its seat-blocking policy has not changed. Where needed, seats could be unblocked in order to get customers to their destinations on the same day.