A Southwest airline plane is seen parked on the tarmac at Phoenix Sky Harbor International Airport in Phoenix, Arizona, Sept. 19, 2016. DANIEL SLIM/AFP/Getty Images

Southwest Airlines' computer system communicating with the Federal Aviation Administration was down Friday, causing flight delays. Several passengers took to Twitter to complain about boarding delays.

"Highly irritated #southwest flights are grounded due to outage - so much for the start of my first kid-free weekend trip," one Twitter user wrote.

Replying to the passengers from its official Twitter account, the airline apologized for the inconvenience caused and said their team was working on it.

"We're truly sorry for the delays this morning. Our Network Operations Control Team is aware of the issue and are working diligently to get you on your way as quickly as possible. We appreciate your patience while we work through this," the airline wrote.

This is a developing story.