Delta Air Lines flights gradually resumed this afternoon after an outage hit its computer systems, grounding planes and stranding passengers at airports around the globe.

Atlanta-based Delta, the second-largest US carrier by passenger traffic, said it had cancelled about 300 flights after a power outage that began around 7.30am (Irish time) in Atlanta.

The power interruption caused a "system-wide outage," Delta said. 

By 3.30pm this afternoon, it said about 800 of its nearly 6,000 scheduled flights had taken off.

The Dublin Airport Authority confirmed that an Atlanta bound Delta flight departed from Dublin at around 3pm having originally been due to leave at 10.10am.

A New York bound flight was due to take off at 3.20pm. It's scheduled time of departure was 11.45am.

Fault in Delta equipment

Georgia Power said a piece of Delta equipment failed and caused the airline's computer systems to lose power.

The problem at Delta did not affect other Georgia Power customers, and there was no area-wide outage, utility spokesman John Kraft said.

Like many large airlines, Delta uses its proprietary computer system for its bookings and operations, and the fact that other airlines appeared unaffected by the outage also pointed to the company's equipment, said independent industry analyst Robert Mann.

Critical computer systems have backups and are tested to ensure high reliability, he said. 

It was not clear why those systems had not worked to prevent Delta's problems, he added.

A Delta spokeswoman declined to comment when asked about backup systems.

Delta's flight information was not showing correctly on Delta's website or on airport information boards, and this could also take time to resolve, the carrier said in the latest update.

Delta is a member of the SkyTeam alliance alongside airlines including Air France-KLM. 

It also partners for transatlantic flights with Virgin Atlantic, which said its flights were operating normally but cautioned that passengers should check tickets in case their flight was due to be operated by Delta as part of a code-share agreement.

Delta said passengers booked for travel between August 8th and 12th would be entitled to a refund if their flight is cancelled or significantly delayed.

Delta shares were up marginally in midday trading on Wall Street.