[Source: Reuters]

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.

Shares of the biggest U.S. airlines, including American Airlines (AAL.O), opens new tab, Delta Airlines (DAL.N), opens new tab and United Airlines (UAL.O), opens new tab, turned negative after Trump’s social media post.

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.

FlightAware, a flight-tracking website, said by 3 p.m. ET (1800 GMT), 5,825 flights were also delayed Monday after 2,950 flights were canceled and nearly 11,200 delayed Sunday in the single worst day for flight disruptions since the government shutdown began on October 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 October 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.

Trump scolded controllers who have taken time off and called those who have continued to work “GREAT PATRIOTS.” Representative Rick Larsen, the top Democrat on the House of Representatives committee overseeing the FAA, said the controllers “deserve our thanks and appreciation, not unhinged attacks on their patriotism.”

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