United Parcel Service Inc. (UPS) said on Tuesday it would require employees in certain U.S. locations to be vaccinated when they return to the office, as COVID-19 cases rise across the United States due to the fast-spreading Delta variant.

The decision comes after the U.S. drug regulator on Monday granted full approval to the Pfizer/BioNTech vaccine, a move that is expected to spur a string of similar mandates from corporate employers. Facebook Inc, Lyft Inc and several other major companies have already mandated vaccination.

"In certain U.S. office locations where employees have been working remotely, UPSers must be vaccinated when they return to office," the company said in an emailed statement.

"Those office environments are very different than our operating facilities, which have been safely staffed in-person since the beginning and throughout the pandemic."

Rival FedEx Corp said it would continue to strongly encourage employees to get vaccinated but has not yet made vaccinations compulsory.

Demand for UPS and FedEx's services has boomed during the pandemic as people staying at home shop more online. (Reporting by Kannaki Deka in Bengaluru; Editing by Aditya Soni)