Four cases of Zika infection in Florida are very likely to have been caused by mosquitoes there, the State Department of Health said Friday — the first documented instances of local transmission in the continental United States.

“Zika is now here,” Dr. Thomas R. Frieden, the director of the Centers for Disease Control and Prevention, said at a news briefing.

The C.D.C. and Florida officials said that for now, the area of concern is limited to one square mile in the Wynwood neighborhood of Miami, a gentrifying area with restaurants and art galleries just north of downtown.

Health authorities are not advising people to stay away from the neighborhood, Dr. Frieden said. The four people appear to have been infected in early July; since then, mosquito control efforts have been stepped up in the area, and additional cases have not been identified.