(Reuters) - As Hurricane Irma bore down on the southern United States on Wednesday, airlines adjusted flight schedules, made cancellations and assured passengers they would not have to pay unusually high fares ahead of the storm’s arrival.

Irma, the second powerful hurricane to approach the United States in as many weeks, was expected to make landfall in Florida by the weekend. It had already pummeled islands in the Caribbean with rain, pounding winds and surging surf by Wednesday.

While the storm’s precise trajectory remained uncertain, airlines preemptively canceled flights in the likely affected regions and put in place travel waivers for customers to reschedule plans.

American Airlines, the largest U.S. carrier by passenger traffic, said on Wednesday it would begin winding down operations in south Florida, including Miami, Fort Lauderdale and West Palm Beach, on Friday. Miami-bound flights arriving on Friday from Europe and South America were canceled.

American, Delta Air Lines and JetBlue all announced fare caps on flights out of Florida - $99 on JetBlue and American and $399 on Delta - for residents trying to get out of the storm’s path.

“We want those trying to leave ahead of the hurricane to focus on their safe evacuation rather than worry about the cost of flights,” JetBlue spokesman Doug McGraw said.

Airlines have been criticized in the past for raising prices in the wake of deadly episodes and, as Irma approached, some social media users accused carriers of engaging in price-gouging schemes ahead of the dangerous storm.

In response, Senators Richard Blumenthal and Edward Markey called on the U.S. Department of Transportation on Wednesday to launch an investigation into potential opportunistic fare hikes by airlines.

“It would certainly be offensive if airlines – who rely on publicly supported infrastructure and have been bolstered by American taxpayers for nearly a century – used this opportunity to impose unconscionable costs on consumers,” they wrote in a letter to Transportation Secretary Elaine Chao.

Accusations of unfair pricing techniques have been investigated in the past, including after a deadly Amtrak derailment in 2015, but U.S. officials said last year they found no evidence of wrongdoing in that instance.

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United Airlines, which took a substantial financial hit when Hurricane Harvey slammed into Texas last week, said it had suspended operations out of San Juan, Puerto Rico, a U.S. territory that was being raked by the very powerful Category 5 hurricane on Wednesday afternoon, and had extended a travel waiver to include cities in south Florida.

In some cases, carriers added flights and upsized aircraft out of Florida and the Caribbean before the storm struck to accommodate as many passengers evacuating the area as possible.

Fort Lauderdale-based Spirit Airlines said it expected its largest operational hub to be affected and planned to move its operations center to Detroit on Thursday evening.

Beyond U.S. airlines, Canadian carriers Air Transat and WestJet Airlines both launched evacuation operations on Wednesday to remove passengers that could be affected by Hurricane Irma in the Dominican Republic, and Air Canada allowed passengers to change flights in impacted areas free-of-charge.

WestJet, Canada’s second-largest carrier, operated rescue flights to Punta Cana and Puerto Plata on Wednesday and could make additional trips to Santa Clara and Cayo Coco, Cuba, on Thursday, airline spokeswoman Lauren Stewart said.

Carnival Cruise Lines, which has major operations in Florida, canceled two of its Bahamas-bound cruises and said it was likely that other schedules would be affected as the storm’s path and impact became more clear.