The rule would apply for flights to India, Brazil, Chile, Peru, Colombia and Argentina, the company said.
The list may extend to China and Taiwan based on potential accommodations received from local governments, according to an internal memo seen by Reuters.
Pilots undertaking flights to these countries need to be fully vaccinated from Aug. 1 and the same rule will apply to flight attendants from July 31.
"We hope that countries come off the list, rather than having to add more countries," the airline said.
Last month, the Chicago, Illinois-based airline had reached a deal with its pilots' union that does not make vaccine mandatory, but provides extra pay to those who receive it.
The agreement also restricted unvaccinated pilots from working on trips to places that require vaccinations.
Airlines in the United States are seeing an increase in travel demand as the rate of vaccination picks up pace and countries ease border restrictions.
(Reporting by Tracy Rucinski in Chicago and Shreyasee Raj in Bengaluru; Editing by Ramakrishnan M. and Arun Koyyur)
