Delta Air Lines Inc. DAL, +3.09% on Monday morning grounded all flights worldwide as a computer issue is leaving passengers unable to check in.

In a post to Twitter, the U.S. carrier said “our systems are down everywhere”, prompting all flights to be grounded. The company further explained in a press release that a computer outage has affected flights this morning.

“Flights awaiting departure are currently delayed. Flights en route are operating normally,” the airline said. A representative from Delta said the company is still trying to understand the nature of the issue. Delta shares weren’t moving in U.S. premarket trade.