WASHINGTON, Oct 1 (Reuters) - The White House is pressing major U.S. airlines to mandate COVID-19 vaccines for employees by Dec. 8 - the deadline for federal contractors to do so - and is showing no signs of pushing back the date, four sources told Reuters on Friday. White House COVID-19 response coordinator Jeffrey Zients spoke to the chief executives of American Airlines (AAL.O), Delta Air Lines (DAL.N) and Southwest Airlines (LUV.N) on Thursday to ensure they were working expeditiously to develop and enforce vaccine requirements ahead of that deadline, the sources said, speaking on condition of anonymity. Large U.S. airlines have a number of federal contracts. President Joe Biden signed an executive order last month requiring federal contractors to mandate COVID-19 shots for employees, with the White House last week setting the Dec. 8 deadline for completing the vaccinations. American Airlines on Friday evening said more than 100,000 U.S.-based employees will need to get vaccinated, but did not specify a compliance date. It added that employees will be able to seek religious or health exemptions to vaccination."While we are still working through the details of the federal requirements, it is clear that team members who choose to remain unvaccinated will not be able to work at American Airlines," Chief Executive Doug Parker and President Robert Isom said in a memo. "We realize this federal mandate may be difficult, but it is what is required of our company, and we will comply." Some airline officials had asked the White House to push back the requirements, signed by Biden last month, until after the busy holiday travel season. Zients urged the airlines "to act sooner than later to ensure as smooth of an implementation process as possible," one source said, and made clear the White House does not intend to relax the deadline. Zients also urged them to look at the United Airlines (UAL.O) vaccine requirement that was announced in August. The three airlines separately confirmed the calls took place but declined to discuss the specifics. Zients did not respond to a request for comment on the calls, first reported by Reuters."Employers should act now to protect their workforce," Zients told a press briefing on Friday, without directly discussing airlines. "More and more companies are stepping up to make vaccine requirements the standard across all sectors." The Civil Reserve Air Fleet (CRAF) is among the federal contracts for major U.S. carriers. The reserve fleet was activated in August in support of the Pentagon, as airlines helped ferry people who have been evacuated from Afghanistan. Biden"s administration notified carriers on Thursday it will seek a modification of CRAF contracts to require vaccinations of airline employees, sources told Reuters. Other government agencies are also expected to seek amendments to contracts with airlines. LABOR SHORTAGE WORRIESThe Allied Pilots Association, which represents 14,000 pilots who fly for American Airlines, last week said that "mandatory vaccinations could result in labor shortages and create serious operational problems for American Airlines and its peers." Some employees of various U.S. businesses have quit rather than comply with vaccine mandates. Two smaller airlines said earlier on Friday they would comply with the vaccine mandate for federal contractors. JetBlue Airways (JBLU.O) said it had "communicated this vaccine requirement to our crew members." Alaska Airlines (ALK.N) said it would comply with the federal contractor vaccine requirements, saying it believes it and other major U.S.  airlines are covered by the executive order. Alaska Airlines said it "means all of our employees, including  certain  contractors and vendors, will be required to be fully vaccinated, or be approved for a reasonable accommodation such as medical conditions or religious beliefs that prevent them from being vaccinated." It added: "The date by which employees must be fully vaccinated has not been confirmed by the government, but it could be as early at Dec. 8." The Federal Acquisition Regulatory Council, which provides guidance to U.S. agencies on contracts and procurement, issued a memorandum on Thursday on incorporating a clause into their solicitations and contracts on vaccines. It is expected to issue guidance on exemptions on Oct. 8, sources said. Separately, the Labor Department will issue an emergencyorder covering more than 80 million private-sector employees to require either regular COVID-19 testing or vaccines. That order is expected this month. Delta said on Friday that 84% of its employees are vaccinated and it continues "to evaluate the administration"s plan." Southwest said it "continues to strongly encourage employees to receive the COVID-19 vaccine." United Airlines said 99.5% of its U.S.-based employees have been vaccinated, excluding those who have sought an exemption. The carrier said only 320 U.S.-based staff are not in compliance with its vaccination policy. United, which in August became the first U.S. carrier to require vaccinations for all domestic employees, had asked staff to provide proof of vaccination by Monday or face termination.