CHICAGO (AP) — United Airlines says it's computer systems are up again.

The airline made the announcement on Twitter more than five hours the systems went down, grounding flights across the country Friday night.

The airline says it's in the process of resuming operations. It did not return calls seeking comment.

As a result of the outage, long lines of passengers formed at check-in counters at Chicago's O'Hare International and Denver International airports, two of United's largest hubs.