New York City Mayor Bill de Blasio said on Monday he planned to issue a COVID-19 vaccine mandate for private-sector employers that will go into effect on Dec. 27, calling it a "pre-emptive strike."

Several indicators on Monday showed the spread of COVID-19 were increasing in the most populous city in the United States, including the percentage of people who are testing positive for the virus, according to data from New York City.

In November, a U.S. appeals court upheld its decision to put on hold an order by President Joe Biden for companies with 100 workers or more to require COVID-19 vaccines, rejecting a challenge by his administration.

(Reporting by Brendan O'Brien; Editing by Doina Chiacu and Chizu Nomiyama) ((brendan.obrien@thomsonreuters.com;))