The outage has also caused the website and app to go down.
Reports suggest United Airlines has a nationwide ground stop related to the outage.
In a series of tweets, the airline apologised and said “our teams are working on this issue now”.
People caught up in the chaos at airports took to social media.
Twitter user Peter J Olmsted posted: “@united Cincinnati United Airlines is saying they are experiencing a nation-wide system outage and cannot do anything with checking-in or boarding flights.”
Another user, Brandi, wrote: “@united what is the reason for the technical difficulties at EWR right now? When will they be resolved?”
Twitter user Chip Watts said: “@united System down? Can’t check bags but welcome to carryon? What about my wife’s bag? Should I just leave her in @BHMAirport?”
Another user, Aaron M. Hurd, added: “@united Is there a system outage affecting boarding and dispatch? DSM airport can’t board the flight due to system issue.”
One more, Michele Sullivan, tweeted: “Apparently @UnitedAirlines North American computer system has crashed. Of course I’m sitting in a United airplane. Aaarrgghhhhh.”
Express has contacted United Airlines for comment.
MORE TO FOLLOW