New increases in the number of coronavirus cases and risky behavior among people at bars, restaurants, and beaches has caused officials in Pennsylvania, New Jersey, and Delaware to warn residents on Monday to follow safety measures amid the ongoing pandemic.

New Jersey Gov. Phil Murphy said indoor dining would not be allowed to begin this week as planned and said the state was “losing ground” in terms of numbers of new cases and hospitalizations, citing data from the last two weeks. Delaware, which had been scheduled to move to its final phase of reopening Monday but postponed it, was warning weekend beachgoers to get coronavirus tests.

Pennsylvania saw a “slight uptick” in new cases over the last week, Gov. Tom Wolf said Monday, and daily case numbers in Philadelphia stayed steady over the weekend rather than declining.

Philadelphia officials are expected to provide an update Tuesday on the city’s potential move to its modified “green” phase, aspects of which may be postponed because case numbers are not falling. The city added nearly 300 new coronavirus cases over Friday, Saturday, and Sunday, keeping in line with recent daily case counts.

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The start-and-stop caution from the region’s officials comes as states nationwide grapple with the delicate and largely unknown calculation for reopening, and people tired of being locked down begin to venture out more.

As some states that reopened faster saw large spikes in infections last week, Pennsylvania and New Jersey were among more cautious states with declining case numbers. But people congregating at restaurants, bars, and beaches have contributed to the new upticks in infections or pauses in reopening.

Murphy said indoor dining, which was set to resume Thursday, will be postponed indefinitely, citing reports of new cases spiking in other states following restaurant openings as well as instances of “knucklehead behavior” at local outdoor restaurants.

“We’ve always said that we would not hesitate to hit pause if needed to safeguard public health,” Murphy said during his daily coronavirus news briefing Monday. “This is one of those times.”

At least 12 other states, including Delaware, have paused reopening plans or put restrictions back in place due to a surge in new cases. Crowds at Delaware beaches over the weekend made health officials there “extremely concerned.” Public Health Director Karyl Rattay said many were observed without masks or not social distancing, including inside bars and restaurants.

That behavior is “a recipe for disaster” and can lead to widespread infection, Rattay said.

New Jersey’s postponement of indoor dining threw the reopening of Atlantic City casinos, which are still permitted to open their doors Thursday, into some uncertainty.

Borgata, which had planned opening to the public next Monday after an earlier invitation-only opening, said it would not reopen without the ability to offer indoor dining.

“We respect the governor’s decision to postpone the reopening of indoor dining in New Jersey to protect the public,” said Borgata parent company, MGM Resorts International, in a statement. “Our guests expect a special experience when they come to our property and if we cannot provide that level of hospitality, we feel it best that we remain closed.”

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On Facebook, Golden Nugget said it would “try everything we can” to open only with outdoor dining: “We would rather get some team members back to work, and provide some experience for those interested, rather then continue to stay closed.”

Joe Lupo, president of Hard Rock Hotel & Casino, said the gambling resort was waiting for details from Murphy’s executive order before issuing an update on operations. “We are disappointed that we cannot provide the experience our guests expect and deserve,” Lupo said.

Murphy said most restaurants had been prepared to start indoor dining safely, but “the carelessness of one establishment can completely undo the good work of many others.”

“It brings me no joy to do this, but we have no choice,” he said.

U.S. Rep. Jeff Van Drew (R., N.J.), whose district includes Atlantic and Cape May Counties, said in a statement he was disgusted by the governor’s decision, “This is Gov. Murphy forcing businesses to die,” he said.

Indoor shopping malls in New Jersey resumed operating Monday for the first time since mid-March, with shoppers lining up outside Cherry Hill Mall before its 11 a.m. opening.

The state’s overall rate of transmission remains low, though the rate, too, has crept up rather than declining since mid-June, Murphy said. In recent weeks, several counties including Camden, Gloucester, Burlington, and Cape May have also reported rising transmission rates.

In Pennsylvania, which reported the largest single-day increase in new cases since June 12 on Saturday, the biggest increases in infections over the last seven days compared with two weeks ago were in Allegheny, Lackawanna, and Lancaster Counties. Philadelphia had a slight increase, Montgomery, Delaware, and Chester Counties had declining rates, and Bucks remained steady.

Philadelphia officials last week said they were seeing an increase in cases in young people that could be attributed to social activity.

In Allegheny County, which includes Pittsburgh, some of the uptick may be linked to an increase in young adults going out to bars and not social distancing, state and county health officials said. County officials suspended the sale of alcohol for on-site consumption at bars and restaurants.

“We know when we have a problem,” said Wolf outside the UPMC Pinnacle Community Osteopathic Hospital in Harrisburg. “We think it’s the bars. I think in the Southeast and Philadelphia, they’re thinking it’s the same thing.”

Other cases in Allegheny County were attributed to out-of-state travel; Wolf said he was not considering an official quarantine for travelers but asked people to stay home for 14 days after returning to Pennsylvania.

The rise in cases suggests people don’t understand what the state’s “green” phase meant, said Anne B. Newman, chair of the epidemiology department at the University of Pittsburgh Graduate School of Public Health.

“I think people took the green to mean that everything was fine and there wasn’t a problem,” she said.

On Monday, U.S. Surgeon General Jerome Adams and Seema Verma, administrator for the Centers for Medicare and Medicaid Services, visited Philadelphia to talk with representatives of long-term-care homes. Verma was concerned about how the state had responded to outbreaks in nursing homes, she said after the listening session.

Verma said Pennsylvania had completed far fewer infection-control surveys at nursing homes than other states on average, but the Department of Health said its count was higher than Verma’s. A spokesperson said the state was trying to reconcile the discrepancy and had completed hundreds of infection-control surveys, although he could not say in how many nursing homes.

On the hospital visit with Wolf, Health Secretary Rachel Levine told doctors and nurses that Pennsylvanians could show “how much they appreciate your hard work” by wearing a face mask when in public.

“When you wear a mask, whether you’re walking on a busy street, whether you’re inside a grocery store or riding transportation,” Levine said, “that is a sign to the whole community that we are in this together.”

Staff writers Tom Avril, Amy S. Rosenberg, Stacey Burling, Rob Tornoe, Sam Wood, and Laura McCrystal, and graphics editor John Duchneskie contributed to this article.