United Airlines on Friday said it notified some 14,000 staff that their jobs would be at risk this spring, while airline unions made a new request to Congress for another $15 billion in government aid to keep employees on the payroll through at least Sept. 30, reports said Saturday.

The new warnings cover every employment group except pilots, United Airlines said. U.S. airlines recalled last month furloughed personnel after Congress renewed federal stimulus for the ailing industry through March 31.

The Chicago-based United cautioned that once a second round of pay package expires on April 1, carriers could be forced to initiate drastic new cuts as the coronavirus crisis continue to deal a heavy blow to global air travel.

Airline companies are legally mandated to notify workers if their jobs are in danger, usually two months in advance. It doesn't mean that these employees will ultimately become jobless, though. United is seeking new voluntary measures to slash its workforce.

United had reactivated around 13,000 staff from forced leave when a $15 billion airline payroll boost was approved in December last year to protect jobs through March.

"Despite ongoing measures to distribute vaccines, customer demand hasn't changed much," United told staff while clarifying the company was keeping a tight watch on demand and working on continued government bail.

For its part, American Airlines Group may release a second tranche of furlough notifications when government assistance expires, Chief Executive Officer Doug Parker said during a meeting to discuss the airline's fourth-quarter loss.

Hawaiian Airlines also issued furlough warnings to employees and said the carrier could let go of up to 900 workers when government aid is depleted. Some American Airlines staff could get similar notifications soon, a flight attendant union memo said.

The Association of Professional Flight Attendants and the Association of Flight Attendants-CWA sent a letter Friday to President Joe Biden and Congressional legislators requesting $15 billion for a third round of payroll aid that would keep jobs until Sept. 30.

Airline union leaders Julie Hedrick and Sara Nelson said, "the alternative now is mass layoffs starting in April."

Meanwhile, United Airlines is offering a new separation program for longer-serving workers in an attempt to further reduce operational costs because staffing levels remain too steep for the outlook in travel demand over the next 12 months.

International travel is seen to be heavily affected until most of the American population is vaccinated and a level of so-called "herd immunity" is attained. It remains unclear when the large-scale nationwide inoculation campaign might achieve this goal.