ATLANTA — Delta Air Lines says ‘limited’ service has resumed but the massive backlog of delayed and cancelled flights in the wake of Delta’s system-wide outage will be causing headaches for travel agents rebooking clients for quite a while.

Delta said the outage began at around 2:30 a.m. ET. Flights that were already en route were operating normally, but others were delayed or cancelled. Pearson International Airport in Toronto and the Montreal-Pierre Elliott Trudeau International Airport were among many that were affected by the airline’s global computer shutdown.

Delta issued a statement apologizing for the inconvenience and said its teams were working to resolve the problem as soon as possible.

It said that flight status systems, including airport screens, were incorrectly showing flights on time, something the company was trying address.

TV footage in the U.S. showed some boarding passes were being written out by hand.

Early confirmation of the troubles first came in an official Delta account that responds to customers via Twitter. The company had said its IT systems were down “everywhere” and “hopefully it won’t be much longer.” Several applications were affected, including the company’s website.

Stephen Smith, 32, of Baton Rouge, La., tweeted “Waiting game at this point” to The Associated Press as his Shanghai-bound plane sat for hours on the tarmac at Tokyo’s Narita airport. Smith took solace in the fact the air conditioning on the plane was working and said it seemed everyone on board was fine.

Delta said travellers will be entitled to a refund if the flight is cancelled or significantly delayed. Travellers on some routes can also make a one-time change to the ticket free of charge.

The latest update on Delta’s website had these details: “Due to a Delta system wide outage travel has been impacted to/from/through all Delta Air Lines served cities. If your flight is cancelled or significantly delayed, you are entitled to a refund. Even if your flight is not cancelled, you may make a one-time change to your ticket without fee if you are scheduled to travel to, from, or through the following destination(s) on Delta, Delta Connection, or Delta-coded flights.” So far the policy applies to all Delta flights on Aug. 8. Tickets must be reissued on or before Aug. 12 and rebooked travel must begin no later than Aug. 12.

With files from The Canadian Press.