Coronavirus infections jumped by 30 percent last month among New York City residents between the ages of 20 to 29, a pattern mirrored in the rest of the country where public officials have attributed most of the new cases to younger Americans.

Between June 13th and June 27th, the number of virus infections in that age group has ticked up from a rate of 26.6 per 100,000 people to 34.6 per 100,000, according to data provided by City Hall. During that same period, cases for those aged 30 to 39 stayed relatively constant at around 30 per 100,000. Recorded infections among those older than 39 have all steadily declined.

Overall, the city's positivity rate for coronavirus has remained relatively flat at 2 percent. And what appeared to be the second time since the crisis began, New York City did not officially record any deaths on Sunday, although the numbers could be revised later. At its peak, around 800 residents across the five boroughs died from the virus in a single day in April.

Mayor Bill de Blasio on Monday cited the uptick in infections among twenty-somethings.

"We can't ever ignore a warning sign," he said.

Separately, he urged residents to wear masks when entering potentially crowded indoor settings like an office or store.

New guidance from the Department of Health released Monday states, "New Yorkers should wear a face covering whenever they are with other people in an indoor setting that is not their home, even if 6 feet of distance can be maintained."

The trend of growing infections among younger adults could portend larger outbreaks, especially amid stories of parties where revelers have been seen maskless and closely congregating. The mayor did not say whether public health officials believed the uptick in infections among twenty-somethings might be due to the ongoing protests against racist policing. So far there has been no established connection between demonstrations and spread of coronavirus.

As infections soar across the country, public health officials have increasingly stressed the use of face masks as the best tool to fighting the virus. New York already has a broad face mask statewide mandate. Under an executive order from Governor Andrew Cuomo, which went into effect in mid-April, all New Yorkers over the age of two must don a face mask or covering when they venture outside and are unable to socially distance at least six feet from others, including on public transit and for-hire vehicles like taxis and Ubers.

Prior to Cuomo's order, de Blasio had advised New Yorkers to wear a face covering whenever they went outside and could expect to be near other people. He also issued guidance that allowed supermarkets and grocery stores to make masks mandatory for customers.

On Monday, the mayor said people should err on the side of caution.

"If you're indoors and there's people around anywhere, even if you're in a big space indoors and there's people in some part of that space, keep that face covering on," he added. "We've talked about before if you were more than 6 feet away from people but what we're finding is people start out more than 6 feet away from people but they end up in contact with them or getting closer to them over the course of the day."

He later added: "If you're at work, a store, even if there seems to be a lot of space, just keep that face covering on all the time. It's a good precaution anytime there are people around, keep it on."