Delta Air Lines said operations were slowly returning to normal early Monday after a systems outage forced the airline to ground all domestic flights Sunday night.

The outage occurred around 6:30 p.m. Sunday when the airline's essential IT systems crashed, prompting significant departure delays and cancellations. The systems were restored several hours later and were back to normal shortly after midnight Monday.

"I want to apologize to all of our customers who have been impacted by this frustrating situation," the carrier's CEO Ed Bastian said in a statement. "This type of disruption is not acceptable to the Delta family, which prides itself on reliability and customer service."

"I also want to thank our employees who are working tirelessly to accommodate our customers."

UPDATE: Systems return to normal; some flight cancellations linger: https://t.co/vrAnpIvQz8

— Delta (@Delta) January 30, 2017

Information to know if you’re flying with us today: https://t.co/Eu9xRGfLnG

— Delta (@Delta) January 30, 2017

Delta said approximately 170 flights scheduled for Sunday were canceled, as well as 110 flights scheduled for Monday, but that list could grow as the day goes on, Delta noted.

In response to the outage, the airline has issued a waiver for travel scheduled on Sunday and Monday, for rebooking by Friday. 

A waiver has been issued for travel scheduled on Jan. 29 and 30, for rebooking by Feb. 3. https://t.co/QWk4cxWsap

— Delta (@Delta) January 30, 2017

Unaccompanied minors will not be accepted for flights through noon ET, Monday, January 30.

— Delta (@Delta) January 30, 2017

Sunday night's outage is the latest incident in a troubling trend for some major airlines. Just one week ago, United Airlines had all of its domestic mainline flights grounded briefly because of a computer-related issue, while Delta was forced to apologize to travelers as recently as August after a computer outage led to thousands of flight cancellations.

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