An additional 2,300 US flights were cancelled Monday as President Donald Trump threatened to dock pay for air traffic controllers who called in sick during the government shutdown.

After Trump ripped absent aviation workers as unpatriotic, the National Air Traffic Controllers Association (NATCA) labor union hailed members working without pay as "unsung heroes" in a statement that called for Congress to immediately end the shutdown.

"Enough is enough," the union said.

The back-and-forth highlights the mounting strain on the aviation industry as the record-breaking shutdown hit day 41, though a compromise bill advancing in Congress was raising hopes of a resolution this week.

The air traffic control system was already under strain prior to the shutdown due to understaffing, and is facing a surge in passengers with upcoming Thanksgiving holiday travel.

Besides Monday's 2,300 cancellations, more than 8,700 flights involving US airports were delayed, according to website FlightAware. And carriers have already scrapped 1,100 flights scheduled for Tuesday, the flight tracker reported.

The Trump administration last week ordered 10 percent reductions in flights at dozens of airports, including some of the nation's busiest, due to "staffing triggers."

Trump took to social media on Monday to threaten that controllers who do not return to work "will be substantially 'docked.'"

"All Air Traffic Controllers must get back to work, NOW!!!" he demanded on his Truth Social platform.

Trump said he was recommending a bonus of $10,000 to the "GREAT PATRIOTS" who did not take time off during the shutdown.

His statement was posted just as the air traffic controller union was concluding a press conference, timed to the second consecutive zero-dollar paycheck for its members.

Union president Nick Daniels called an emerging deal in Congress a "right step in the right direction."

"Air traffic controllers should not be the political pawn during a government shutdown," said Daniels, who has spoken in increasingly dire terms since federal funding first lapsed on October 1.

A NATCA statement released later Monday, following Trump's social media threat, said controllers "deserve our praise."

"This nation's air traffic controllers have been working without pay for over 40 days," the union said. "The vast majority of these highly trained and skilled professionals continue to perform one of the most stressful and demanding jobs in the world, despite not being compensated. Many are working six-day weeks and ten-hour days without any pay."

Democratic congressman Rick Larsen called Trump's comments "nuts."

"The women and men working long hours in air traffic control towers to keep the aviation system running deserve our thanks and appreciation, not unhinged attacks on their patriotism," said the Democrat from Washington state.

Hours later, during a phone-in interview to broadcaster Fox News, Trump doubled down on the bonuses but admitted he was unsure where the funds might come from.

"I don't know. I'll get it from someplace...I always get the money from someplace," the billionaire president said. "Regardless, it doesn't matter."

- Working two jobs -

Prospects for a resolution to the longest shutdown in US history looked brighter on Monday, after enough Democrats in the US Senate joined Republicans to pass a bill to fund the government through January.

However, Daniels noted that after a lengthy shutdown in 2019 it took two and a half months for all controllers to receive back pay.

Meanwhile, "January 30 will loom around the corner," he added, alluding to uncertainty about what will happen after the next spending bill expires.

Daniels was joined at the event by Amy Lark, who works at an air traffic facility in Virginia.

Her family is having to make do without two paychecks, because Lark's husband also works for the agency.

"Yesterday, my kids asked me how long we could stay in our house. Having to answer that question was heartbreaking," said Lark.

The cutbacks are also forcing travelers to adapt.

"It's a little crazy this morning," said Jack Nicks at Miami International Airport, adding he doublechecked to make sure his flight would be ready for takeoff.

"I have other friends that are flying today. They've already had three flight changes. So it's a little rough."