United Airlines will send layoff warnings to 36,000 employees - nearly half its U.S. staff - in the clearest signal yet of how deeply the virus outbreak is hurting the airline industry.

The outlook for a recovery in air travel has dimmed in just the past two weeks, as infection rates rise in much of the U.S. and some states imposed new quarantine requirements.

United officials said Wednesday that they still hope to limit the number of October layoffs by offering early retirement, and that the 36,000 number is a worst case scenario. The notices being sent to employees this month are meant to comply with 60-day warning ahead of mass job cuts.

The furloughs would include 15,000 flight attendants, 11,000 customer service and gate agents, 5,500 maintenance workers and 2,250 pilots.

"The United Airlines projected furlough numbers are a gut punch, but they are also the most honest assessment we've seen on the state of the industry," said Sara Nelson, president of the Association of Flight Attendants. "This crisis dwarfs all others in aviation history, and there's no end in sight."

If United carries through on the notices, furloughs would take effect on or shortly after Oct. 1. United can't lay off workers before then as a condition of the $5 billion in federal payroll aid it began receiving this spring.

The flight attendants' union and other airline labor groups are lobbying Congress to approve another $25 billion in payroll aid to protect jobs through next March. But a senior United executive expressed doubt that Congress would approve the spending in an election year.

"We do not feel like we can count on additional government support to survive," said the official, who spoke to reporters on condition of anonymity.

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United executives said the notices covered 45% of the airline's U.S. staff. Another 1,300 management and support staff will be laid off Oct. 1, the company said. Including international employees, United has a work force of about 95,000.

Air travel plunged about 95% from March 1 until mid-April, then began a slow recovery. The number of U.S. air travelers around the July 4 weekend was the highest since mid-March, but was still down about 70% from a year ago.

In recent weeks, the number of new reported cases of COVID-19 has roughly doubled to about 50,000 a day. New York, New Jersey, Connecticut and Chicago have announced that people arriving from states with high infection rates will have to quarantine, throwing up a new roadblock to travel.

United's traffic at its hub in Newark, New Jersey, has slumped more deeply than the rest of its network since those quarantine rules were announced.

Executives of other airlines have predicted their companies will be much smaller, with fewer employees, in October. Delta Air Lines recently told employees that it will send layoff notices to more than 2,500 of its 14,000 pilots.