Flight disruptions that have marred air travel for millions of people in recent weeks could continue even after the government shutdown ends, airlines and the secretary of Transportation said.

The Senate on Monday night passed a bill that could end the longest federal government shutdown in history, sending it to the House for a vote.

But Transportation Secretary Sean Duffy said Tuesday that won’t be an immediate fix.

“We’re going to wait to see the data on our end before we take out the restrictions in travel but it depends on controllers coming back to work,” Duffy said at a press conference at Chicago O’Hare International Airport.

Duffy also warned severe disruptions over the past few days could get much worse without a deal.

The Senate vote came as staffing shortages of air traffic controllers, who are required to work without their regular paychecks in the shutdown, have delayed or canceled thousands of flights, with issues worsening in recent days. Controllers missed their second full paychecks of the shutdown this week, and some have taken up second jobs and are working with increasing levels of stress, government and union officials have said.

Even if the House passes the bill that will fund the federal government through January, airlines said they will need time to readjust.

“Airlines’ reduced flight schedules cannot immediately bounce back to full capacity right after the government reopens,” Airlines for America, a lobbying group for airlines including Delta Air Lines, United Airlines, American Airlines and Southwest Airlines, said late Monday. “It will take time, and there will be residual effects for days. With the Thanksgiving travel period beginning next week and the busy shipping season around the corner, the time to act is now to help mitigate any further impacts to Americans.”

Airlines will need time to reconfigure schedules and position planes and crews, something they were forced to quickly address with last week’s required flight cuts.

More than 5 million travelers have been affected by airline staffing issues since the shutdown began on Oct. 1, Airlines for America said . The disruptions have sent some passengers looking for alternatives, from buses to rental cars and even private jets.

Last Friday, the Trump administration started requiring commercial airlines to cut 4% of their domestic flights at 40 busy U.S. airports, with larger reductions on the way if the shutdown doesn’t end, as officials blamed the strain on air traffic controllers.

Aviation groups have said that record numbers of travelers are expected for the Thanksgiving period, with the holiday just over two weeks away.

Just over 5% of the scheduled 22,811 U.S. departures were canceled on Tuesday, a relatively light day for travel generally, according to aviation data firm Cirium. That’s down from an 8.7% cancellation rate on Monday, or 2,239 flights, and 2,633 cancellations on Sunday, or 10.2% of the schedule. Delays had also piled up with staffing shortages and bad weather at major hubs, including Chicago O’Hare.

The shutdown, like the one in late 2018 to early 2019, has thrust aviation’s strains into the spotlight. The previous shutdown, however, ended hours after a shortfall of air traffic controllers snarled air traffic in the New York area.

Aviation groups on Tuesday urged lawmakers to not only end the shutdown but to provide more Department of Transportation funding to help modernize air traffic control and hire more controllers, who were in short supply even before the shutdown began.

“The government shutdown has disrupted that work and slowed the strong momentum we have built for modernization,” the Modern Skies Coalition, which includes major airline, airport and aerospace groups such as Boeing, GE Aerospace and others, as well as labor unions, wrote in an open letter to Congress.

President Donald Trump on Monday threatened to dock pay of air traffic controllers who are absent. “All Air Traffic Controllers must get back to work, NOW!!!,” he wrote in a post on Truth Social, adding that he would recommend $10,000 bonuses for any air traffic controllers who weren’t absent during the shutdown.

Duffy said he supported Trump’s idea and that he was concerned about the dedication and “patriotism” of controllers who haven’t shown up for work. “If we have controllers who systemically weren’t doing their job, we will take action,” he said.

Duffy said controllers would receive about 70% of their pay within two days of the shutdown ending.

A day earlier, Nick Daniels, president of the National Air Traffic Controllers Association union, said it took about 2½ months before the workers were made whole in the shutdown that ended in 2019.

Duffy said the shutdown has made air traffic controller staffing more challenging, with 15 to 20 of them retiring a day instead of around four retiring a day before the government closure. He said the country is roughly 2,000 controllers short of what the system needs.

“The job of keeping aviation safe and secure is tough every day, but forcing federal employees to do it without pay is unacceptable,” the Modern Skies Coalition wrote in its open letter. “We owe public servants at the Federal Aviation Administration (FAA) and other agencies supporting aviation, like the National Transportation Safety Board, the Transportation Security Administration and Customs and Border Protection, a debt of gratitude and a swift ending to this shutdown.”