U.S. President Joe Biden announced on Thursday that passengers flying to the U.S. will need to show a negative Covid-19 test performed within one day of departure as the spread of the Omicron variant continues to raise concerns.

The new rule, certain to be controversial, goes into effect during the week of December 6. It comes after the country’s first official Omicron variant case was detected in California, discovered in a fully vaccinated patient who had recently returned from South Africa.

The White House did not specify whether the required test should be a PCR, or whether it could be a rapid test, in its statement.

In addition, Biden extended the federal mask mandate on flights, trains, and other forms of public transport through March 18, 2022. The mandate was previously set to expire on January 18.

The administration called the new testing rules and extended mask mandate necessary to “protect the American people against this variant and to continue to battle the Delta variant during the winter months when viruses tend to thrive.” Only about 60 percent of Americans are vaccinated against Covid-19, which is low