United Airlines is firing employees over noncompliance with its vaccine mandate.

United Airlines is terminating about 600 employees for refusing to comply with its vaccination requirement, the company said in a memo sent to staff on Tuesday.

“This was an incredibly difficult decision but keeping our team safe has always been our first priority,” the airline said in the memo.

The company said on Wednesday that it had already begun its termination process for its U.S.-based employees. Workers losing their jobs because of noncompliance with the mandate make up less than 1 percent of the airline’s U.S. work force of 67,000.

“We will work with folks if during that process they decide to get vaccinated,” said a spokeswoman at United Airlines, which did not give a timeline for the termination process.

In early August, the airline announced that all employees would be required to provide proof of vaccination within five weeks of a vaccine’s full approval by the Food and Drug Administration or by Oct. 25, whichever came first. The F.D.A. in late August granted full approval to Pfizer-BioNTech’s coronavirus vaccine for people 16 and older. United had also said it would fire employees who did not follow the new policy.

Other airlines have taken different measures to encourage employees to get inoculated. Delta Air Lines announced last month that it was adding a $200 monthly surcharge on its health care plan for employees who were not vaccinated. The company has also said that it requires new employees to be vaccinated, but that existing employees are exempt. American Airlines said it was “not putting mandates in place” for employees or customers.