Speaking at a White House briefing, officials said that anyone who recently left New York should self-quarantine at home for 14 days to prevent the virus from infecting those in other regions.

“We’re starting to see new cases across Long Island that suggest people have left the city,” the White House coronavirus coordinator, Dr. Deborah Birx, said at the briefing.

Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, said that New Yorkers who were “understandably” trying to leave for places like Florida needed to make sure they were not “seeding” the rest of the United States.

“When they go to another place, for their own safety, they have to be careful,” Dr. Fauci said.

Vice President Mike Pence repeated the advice, calling New York City a “high-risk” area and asking New Yorkers to avoid nonessential travel.