Shortly after 8 a.m., there was a report of a fire in Terminal 1 in the pedway tunnel that connects Concourses B and C for United Airlines Operations, according to United.
"We came in and started smelling smoke and then from there we were told to wait," said Nikkia Glover.
Power was shut off in the affected area and passengers were not allowed to enter the area for their protection, the airlines said.
Shuttle buses were deployed to transport passengers to the B and C concourses for their flights, according to United.
United passengers delayed after fire shuts down O'Hare pedway tunnel
"There was huge inconvenience as you came up from the lower tunnel up into area B where the escalators were shut down," said John Connick of Waukegan. "People trying to come down were all having to come down the stairs at the same time as people trying to come up the stairs so that was a bit of a nightmare."
Katie Suvada was trying to catch a flight to Florida when the fire broke out. She got stuck in a crowd of people trying to make their way through Terminal 1.
United passengers delayed after fire shuts down O'Hare pedway tunnel
The Chicago Fire Department put out the fire at around 9:30 a.m., officials said.
Glover said she had to wait a couple of hours but she said she was thankful United stopped all the flights so people could catch their connecting flights.
"Very efficient about it too, they were on top if it," said Glover.
"It inconvenienced a little bit but I understand safety is first," said Ken Dale Quarles.
No injuries were reported.
The cause of the fire is under investigation.