Tens of thousands of Delta Airlines passengers around the world were stranded Monday by a computer outage that the company said had grounded all its flights.

It blamed a power failure at its hub in Atlanta.

"Delta has experienced a computer outage that has affected flights scheduled for this morning," the company said in a statement.

"Flights awaiting departure are currently delayed. Flights en route are operating normally," the Atlanta, Georgia-based US airline said.

Delta urged travellers to check the status of their flights "while the issue is being addressed."

Delta said in a later statement the breakdown was caused by a power outage in Atlanta that began at around 2:30 am.

It said large-scale cancellations were expected Monday.

On Twitter, Delta representatives urged patience and apologized as they dealt with a flood of messages from concerned passengers.

"All our flights are grounded due to the outage," read one tweet.

"We're experiencing a system wide issue," read another.

Lines of passengers were backing up at Delta ticket counters at US airports.

At Los Angeles, passengers on a flight to New York had to get off their plane and return to the terminal, NBC News reported, while some people slept near departure gates at Las Vegas.