Washington, April 19 : Thousands of flights of Southwest Airlines were delayed across the US following a technical glitch that forced the temporary grounding of all its planes. According to aviation tracking company Flight Aware, more than 5,400 flights were delayed into or out of the US on Tuesday, while over 100 were cancelled, reports the BBC. Confirming the development, the Federal Aviation Administration (FAA) said that "Southwest Airlines experienced a technical issue with one of their internal systems", adding that all departures had been paused at the airline's request while the Dallas-based carrier addressed the issue. "The pause has been lifted and their service has resumed," the FAA said. As services resumed later Tuesday, the low-cost carrier said on social media: "We work to restore operations and accommodate disrupted customers as quickly as possible." Southwest Airlines has been under fire after "staff shortages" during last year's Christmas holiday season led to 16,700 flight cancellations and disrupted travel plans for two million customers. At the time, the US Department of Transportation said it would investigate whether customers were subject to "unfair and deceptive" practices. /IANS