Since downgraded to a tropical storm, Irma made landfall in Florida Sunday as a Category 4 hurricane, creating a wide swath of damage that grounded commercial air travel from Miami to Jacksonville and beyond.
"We're looking to see when it makes sense to resume operations, but for now, we're not operating any flights in and out (of Florida), with the exception of some relief and humanitarian aid," Charlie Hobart, a spokesman for Chicago-based United, said Monday.
FlightAware reported 12,678 delayed flights and 4,461 canceled flights across the U.S. as of early Monday afternoon, with Miami, Orlando, Fort Lauderdale, Tampa and Atlanta's Hartsfield-Jackson International accounting for the bulk of storm-related cancellations.
Southwest and American airlines had the most canceled flights Monday, according to FlightAware, with United less affected by Irma than by Hurricane Harvey, a category 4 hurricane which swept into Houston off the Texas Gulf Coast two weeks ago.
Houston is United's second largest hub after Chicago.
"Orlando is one of our largest nonhub airports," Hobart said. "That being said, our presence in South Florida is quite limited."
United suspended operations in South Florida Friday afternoon, and at other Florida airports on Saturday. The airline added extra flights beginning Thursday to help passengers get out of the state before Irma arrived.
American, which was relatively unaffected by Harvey, is taking a bigger hit from Irma, as Miami is one of its larger hubs.
Dallas-based American had 756 canceled flights as of Monday afternoon, according to FlightAware
American removed all of its planes from Florida in advance of the storm, parking them at distant airports, including Chicago's O'Hare International.
"It's not just because we don't want our aircraft to get damaged, but it's also easier to resume operations once the airport reopens," said Ross Feinstein, a spokesman for American.
An American Boeing 757 was scheduled to take off from O'Hare to Miami on Monday afternoon carrying supplies for the hurricane relief effort, as well as Chicago-based American employees to restart operations there.
"This will give some of our Miami-based employees who have suffered damage from the hurricane time to take care of their homes and families, while still allowing us to get the operation up and running as soon as possible for our customers," American spokeswoman Leslie Scott said in an email Monday.
Operations at major Florida airports are canceled Monday, with flights expected to resume at many, including Tampa International, on Tuesday.
"At Tampa International, we saw sustained winds of 52 mph with a peak gust of 66 mph," Janet Zink, an airport spokeswoman, said in an email. "An early look shows minimal impact to the airfield, terminal and construction sites."
As with Harvey, airlines are partnering with relief organizations to help people impacted by Irma.
In addition to flying in supplies, American is offering members of its frequent flyer program 10 miles for every dollar donated to the Red Cross through Sept. 24, with a $25 minimum donation.
United also is offering bonus miles for passengers who donate at least $50 to the Red Cross and other relief organizations, with the airline pledging to match the first $100,000 raised.
Just 16 days apart, Harvey and Irma mark the first time two Atlantic Category 4 hurricanes have made U.S. landfall within the same year, according to the National Oceanic and Atmospheric Administration.
For United, marshaling resources for Harvey helped the airline respond more quickly to Irma.
"Clearly we dealt with Houston and we were prepared, more so, in terms of how we would pivot to our operation in South Florida," United spokesman Hobart said.