New York City Mayor Bill de Blasio on Monday said the city is seeing a surge in positive cases of coronavirus in individuals between the ages of 20 and 29, calling it a “problem” that needs to be addressed.

De Blasio, during a press conference on Monday, said the city – which was an early epicenter of the COVID-19 pandemic in the United States – is seeing a decline in cases. The mayor reported, for the first time, no new COVID-19 deaths in a 24-hour period, but warned of a new “trend” of positive diagnoses for younger adults.

CUOMO SAYS GEORGE FLOYD PROTESTERS SHOULD 'ASSUME' THEY ARE 'INFECTED' WITH COVID-19

“We are seeing today, even though we’ve seen a huge reduction in the virus in this city, we do see one trend that is worrisome for young adults,” de Blasio said, noting that the infection rate is rising in the city for individuals between the ages of 20 and 29.

De Blasio noted that there are “some increases” in people in their 30s, but that the “20- to 29-year-old” group is where there is “a problem.”

“We see a problem,” he said. “We need to address it.”

De Blasio said that the city would "double down" on efforts to reach younger adults, saying they would be implementing "digital media messages from influencers, a night of action outreach all over the city at outdoor spots where people gather mobile testing vans, mass giveaways."

"We're going to do everything to reach younger adults to remind them they're not impervious," de Blasio said. "And I think many of us can remember when we were in that age range, you feel all too often impervious. But everyone everyone is susceptible is disease. And of course, everyone, unfortunately, could spread it to someone else they love."

He added: "So we're going to double down on making sure that younger adults really follow these precautions that have worked so well for all of us."

De Blasio went on to say that the city was issuing “new guidance,” calling on people to “keep face coverings on” while indoors and people are around.

“If you are at work or in a store, you must keep your face covering on all the time,” de Blasio said. “A good precaution any time people are around … keep it on.”

ANTI-POLICE DEMONSTRATIONS MAY HAVE SPARKED NEW CORONAVIRUS CASES, SOME CITIES NOW ACKNOWLEDGE

De Blasio’s comments come after weeks of protests across the city in response to the death of George Floyd in police custody in May.

Last week, Fox News asked de Blasio’s office whether it believed those protests could have caused any sort of increase in COVID-19 cases in the city – something they denied.

"Based on our health indicators, which measure hospital admissions, number of people in ICU and percentage of New Yorkers testing positive, we have seen no indication of an uptick in cases," Avery Cohen, de Blasio's deputy press secretary, told Fox News.

Cohen added that the number of New Yorkers testing positive "remains at a steady 2 percent, far below the nationwide average," and that, "at this time, we do not believe there has been a resurgence in cases related to the protests, which reached their peak over a month ago."

CDC 'BEST ESTIMATE' IS 40 PERCENT OF COVID-19 CASES ARE ASYMPTOMATIC

Last month, New York Gov. Andrew Cuomo  warned that those who had been participating in protests should “assume” that they are “infected.”

“If you were at one of those protests, I would, out of an abundance of caution, assume that you are infected,” Cuomo said last month. “One person, one person can infect hundreds if you were at a protest.”

He added: “You went to a protest, get a test. Tell people, act as if you may have been exposed.”

As of Monday, New York City reported more than 224,000 positive cases of COVID-19 since the pandemic began and more than 22,750 deaths.

Fox News' Gregg Re contributed to this report.