Aug 13 (Reuters) - International Business Machines Corp (IBM.N) said on Friday that it would allow only fully vaccinated U.S. employees to return to offices, which are set to open from Sept. 7, given the rapid spread of the Delta variant of COVID-19. "We will still open many of our U.S. sites, where local clinical conditions allow, the week of Sept. 7. However, the reopenings will only be for fully vaccinated employees who choose to come into the office," Chief Human Resources Officer Nickle LaMoreaux said in a memo sent to employees. The resurgence of COVID-19 cases in the United States due to the Delta variant and the new guidance from the U.S. Centers for Disease Control and Prevention (CDC) that requires fully vaccinated individuals to wear masks have led companies to change their plans on return to office, vaccinations and masking. The technology firm also asked its employees to get fully vaccinated, joining other big techs to fight the spread of the virus. Earlier on Thursday, Facebook Inc (FB.O) has pushed back its office return date for all U.S. and some international employees until January 2022, while AT&T Inc (T.N) said it will require management employees to be vaccinated before entering a work location.