NEW YORK - Mayor Bill de Blasio says he has serious concerns as the city's daily coronavirus positivity rate has now risen to over three percent for the first time in months.

It comes as city public school students return to classrooms for blended in-person learning. The mayor has said he will close school buildings if the city's positivity rate remains at three percent or above for a week.

During his daily briefing, the mayor said while the city's current seven-day average is 1.38 percent, the rise in the daily rate is alarming.

"We need a clear picture of what's happening in the city," de Blasio said.

The mayor says the uptick is focused on nine specific zip codes in Brooklyn and Queens. They include Gravesend/Homecrest, Midwood, Kew Gardens, Edgemere/Far Rockaway, Borough Park, Bensonhurst/Mapleton, Gerritsen Beach/Homecrest/Sheepshead Bay, Flatands/Midwood.

In those, the mayor says there will be increased community outreach and rapid testing centers set up.

The mayor says the city will begin two new enforcement measures in response to the rise in positive cases.

First, anyone not wearing a face covering wil be reminded and offered one. Those who still refuse will be fined.

Secondly, the city Department of Health will order all private schools and child care centers to close if they don't meet DOE health and safety standards.

If something doesn't change soon, the mayor says further drastic actions may be needed.

"If necessary we will have to prohibit gatherings, except for very small gatherings. If necessary we will have to close non-essential businesses, no one wants that to happen if it can be avoided, if it does have to happen we would obviously try and target it as carefully as possible. But is a situation at this point that is very serious and we have to have all options on the table," De Blasio added.

The mayor said community institutions, including houses of worship, could be included in the shutdowns.

The city's public elementary schools reopened for in-person learning Tuesday morning, three weeks after the originally scheduled start.

Mayor de Blasio and Schools Chancellor Richard Carranza were there to welcome students and staff back at PS 188 on the Lower East Side.

The introduction back to in-person learning will be done in three phases.

Elementary school kids are back Tuesday. Middle and high school students are set to return Thursday. Pre-K, 3-K and special needs students are already back in the classroom.

Some students, however, are still taking part in remote instruction. 

Parents can opt out of the blended learning model and go fully remote at any time.

Those doing 100 percent remote learning can opt in to the blended model in November.