NEW YORK — New York’s public schools will close temporarily to combat a rise in coronavirus cases, its mayor announced Wednesday, dealing a blow to the city’s recovery.

Bill de Blasio said the schools would shut from Thursday “out an abundance of caution” after the city recorded a seven-day average positivity rate of three percent.

“We must fight back the second wave of COVID-19,” he wrote on Twitter.

The three percent threshold had been agreed between the city government and teaching unions when New York’s schools began reopening in September.

New York City is the largest school district in the United States with 1.1 million students.

It has been the only major city in the United States to commit to offering in-person classes this fall as part of a hybrid system that included online learning.

Many cities, such as Chicago, Houston, Los Angeles, Philadelphia and Miami, opted instead for the virtual model.

New York’s closures are controversial with testing showing that schools are not the source of soaring cases. The positivity rate in schools is just 0.19 percent, Richard Carranza, the head of New York public schools, told principals in an email.