Amazon.com has ordered all US employees to wear a mask at work regardless of their vaccination status, as the highly infectious Delta variant of COVID-19 sweeps the country.

Companies across the United States have tightened their defenses against the virus, after the Delta variant forced the US public health agency to reverse course and insist on even fully vaccinated individuals wearing masks.

"We are monitoring the situation closely and will continue to follow local government guidance and work closely with leading medical healthcare professionals, gathering their advice and recommendations as we go forward to ensure our buildings are optimized for the safety of our teams," an Amazon spokesperson said in a statement on Friday.

Last month, big techs including Alphabet's Google and Facebook asked US employees to get vaccinated to step into offices, while Twitter said it was shutting its reopened offices in the country.

Amazon also extended its work-from-home dates for US employees till January 3.

Earlier this week, coronavirus cases worldwide surpassed 200 million while US cases stood at 35.62 million on Thursday, according to a Reuters tally.

Bloomberg News first reported Amazon's change in masking policy on Friday.

© Reuters