Aug 6 (Reuters) - Low-cost carrier Frontier Airlines said on Friday all its employees will be required to be fully vaccinated against COVID-19 by Oct. 1 in view of rising coronavirus cases.

Companies across the United States are increasingly reinstating masking requirements and mandating vaccination to contain the highly transmissible Delta variant.

The airline said employees who choose not to be vaccinated, or are unable to, will be asked to provide proof of a negative COVID-19 test on a regular basis.

"As we continue to watch the rapid increase of new COVID-19 cases across the United States caused by the Delta variant, I am concerned for the well-being of our team members, their families and friends," Chief Executive Officer Barry Biffle said.

"The good news is that the vast majority of our employees have already taken this important step and have gotten vaccinated."

Earlier in the day, United Airlines Inc became the first U.S. airline to require COVID-19 vaccinations for all domestic employees.

Frontier Airlines is also encouraging flyers to get vaccinated with certain incentives through its Friends Fly Free program. (https://bit.ly/3juxjtJ). (Reporting by Shreyasee Raj in Bengaluru; Editing by Shinjini Ganguli)