Delta Air Lines (DAL) U.S. domestic flights were operating again late Sunday night, after being grounded for more than two hours due to a systems outage that resulted in departure delays and cancellations.

But approximately 150 flights have been cancelled, with more expected according to a Delta statement, though the airline warned not all delays and cancellations were being reflected on their systems.

"I want to apologize to all of our customers who have been impacted by this frustrating situation," Delta CEO Ed Bastian said in a statement.

"This type of disruption is not acceptable to the Delta family who prides itself on reliability and customer service. I also want to thank our employees who are working tirelessly to accommodate our customers," said Bastian.

Delta said a waiver has been issued for travel scheduled on Jan. 29 and Jan. 30; passengers have to rebook by Feb. 3. The airline added unaccompanied minors were not going to be allowed on board flights through noon ET on Monday, Jan. 30.

International flights were exempt from the halt.

Passengers stranded in airports took to social media, where a representative on Delta's official Twitter page told users the systems were down and that its IT department was working to rectify the situation.

Last week, a computer problem forced United Airlines to ground all domestic flights for about an hour.

This story was updated with additional information from Delta.