ATLANTA (NewsNation Now) — The Centers for Disease Control and Prevention (CDC) has started offering free COVID-19 tests for passengers arriving from certain southern African countries in an effort to boost detection of the new omicron variant.

The U.S is beefing up testing surveillance at arrival terminals at four international airports: Hartsfield-Jackson Atlanta, John F. Kennedy in New York, Newark Liberty in New Jersey and San Francisco. The tests are voluntary.

The news comes as the first U.S. case of the omicron variant has been identified in California. Dr. Anthony Fauci said the person traveled from South Africa on Nov. 22 and tested positive Monday. 

Passengers who agree to be tested on arrival are also given additional at-home tests and they are asked to send their test in after three to five days.

The CDC said it is also working with airlines to gather contact information for passengers who recently spent time in Botswana, Eswatini, Lesotho, Malawi, Mozambique, Namibia, Zimbabwe or South Africa for contact tracing purposes. Delta, United and American airlines have all agreed to comply with the CDC order.

President Joe Biden has hinted in the last few days that he is considering stepping up new restrictions on international travel in an effort to stop the spread of the variant. The United Kingdom, European Union, Canada and several other countries have announced travel restrictions from southern African countries, as well.

The new requirements could include pre-departure testing closer to the time of a flight, additional post-arrival testing and self-quarantines.

Cases of the omicron variant have been reported in more than 20 countries and territories so far.