President Donald Trump on Monday demanded air traffic controllers return to work as travelers endured another day of flight cancellations, which the administration ordered to manage staff shortages during the government shutdown.

Threatening to curtail the pay of any controller who did not go back, Trump said he would award those who have not taken time off during the 41-day shutdown $10,000 bonuses and would welcome the resignations of the rest. 

“All Air Traffic Controllers must get back to work, NOW!!! Anyone who doesn’t will be substantially ‘docked,'” Trump wrote on social media. “REPORT TO WORK IMMEDIATELY.”

The shutdown, the longest in U.S. history, has forced 13,000 air traffic controllers and 50,000 Transportation Security Administration agents to work without pay. Some are absent as they work second jobs or cannot afford child care.

Some 20% to 40% of controllers have been absent on any given day at the 30 biggest U.S. airports during the shutdown, the FAA said last week.

Officials said it was unclear how the White House could deny pay under the controllers’ union contract once the government reopens, as Trump threatened, or how the president would pay for the proposed $10,000 bonuses. 

Airlines canceled nearly 2,000 flights on Monday, and the number was set to rise as the FAA ordered flight cuts to step up to 10% on Friday. A winter storm in Chicago was also disrupting air travel.

On Sunday, 2,950 flights were canceled and nearly 11,200 delayed in the single worst day for flight disruptions since the government shutdown began on Oct. 1. 

 Staffing issues worsened over the weekend and the number of air-traffic control centers with staff shortages rose to 81 on Saturday, the peak since Oct. 1, Transportation Secretary Sean Duffy said Sunday.

Asked about Trump’s comments, National Air Traffic Controllers Association President Nick Daniels said Monday controllers would appreciate any recognition. “We’ll work with the administration …. Air traffic controllers will continue to show up during this shutdown,” he said.

Airlines urged quick approval of a bill the U.S. Senate voted to advance on Sunday that would reopen the government. It was unclear when Duffy would lift the flight restrictions.

“The government shutdown must end and so must the disruption caused to our customers and the federal employees who are being forced to work without pay,” Southwest CEO Bob Jordan said.

American Airlines said more than 250,000 customers’ flights were canceled or delayed over the weekend. “This is simply unacceptable and everyone deserves better,” American Chief Operating Officer David Seymour told employees.

 The FAA instructed airlines to cut 4% of daily flights starting last week at 40 major airports. That is scheduled to rise to 6% on Tuesday and then hit 10% on Friday.

Even before the shutdown, the FAA was about 3,500 air traffic controllers short of targeted staffing levels. Many had been working mandatory overtime and six-day weeks. Duffy has sought to retain controllers who could retire, speed hiring and undertake a $12.5 billion overhaul of air-traffic control systems.