United Airlines said on Friday it warned some 14,000 employees might be furloughed, and aviation unions made a new request to Congress and President Joe Biden for another $15 billion in government assistance to keep workers on the payroll through at least September 30.

The Chicago-based airline warned that once a second round of payroll support expires on April 1, airlines could be forced to make drastic new cuts as the coronavirus pandemic has slashed demand for air travel.

United had recalled 13,000 employees from furlough when a $15 billion airline industry payroll package was passed in December to protect jobs through March.

"Despite ongoing efforts to distribute vaccines, customer demand has not changed much," United told employees, while saying it was monitoring demand and advocating for continued government support.

The $15 billion in December helped bring back more than 32,000 airline employees and followed a $50 billion package in March for passenger airlines divided between payroll assistance and low-cost government loans.

Two union leaders representing 75,000 flight attendants wrote to congressional leaders seeking quick action to extend the payroll support program "with $15 billion to protect jobs" through September 30 or later.

Union leaders Sara Nelson and Julie Hedrick added, "The alternative is mass layoffs starting in April."

American Airlines, which had furloughed 19,000 workers in October, did not immediately comment on Friday on whether it would issue new notices of potential layoffs.

Hawaiian Airlines said earlier it had issued furlough warnings to 900 employees.

United's Friday memo said, "we are all working hard toward the day when we can bring back our furloughed co-workers permanently."

American Airlines chief executive Doug Parker said on Thursday that "April 1 is approaching and demand hasn't gotten much better... So we are definitely going to need to address this, unless demand starts to pick up."

Parker said the company's unions "are already talking to the administration in Congress about this... We would obviously be supportive of that."