Coronavirus cases in New Jersey, an early epicenter of the pandemic, are on the rise again, doubling over the last month to an average of more than 900 new positive tests a day, a worrisome reversal of fortune for a state that had driven transmission rates to some of the nation’s lowest levels.
After an outbreak several weeks ago in a heavily Orthodox Jewish town near the Jersey Shore, cases are now rising in counties across the state, driven, officials say, by indoor gatherings.
The state’s health commissioner has said there are signs of “widespread community spread” for the first time since New Jersey successfully slowed the spread of a virus that has claimed the lives of more than 16,000 residents. A small, densely packed state, New Jersey has the highest virus fatality rate in the country.
Gov. Philip D. Murphy said Monday that residents should refrain from all but necessary out-of-state travel.
“The numbers are up,” Mr. Murphy said. “They’re up — up and down the state.”
Under a quarantine policy adopted by New Jersey, New York and Connecticut, New Jersey now exceeds the threshold — an average of 10 cases for every 100,000 residents for seven days — used to determine which states should be included in the travel advisory. Thirty-eight other states are on the list.
The uptick comes as other parts of the Northeast and states across the country are confronting similar surges of infections and hospitalizations as the pandemic stretches into its eighth month, with a death toll that now exceeds 219,000, according to a New York Times database.
Mr. Murphy, who has been conservative in allowing the state to reopen, said he would consider targeted shutdowns to curb the spread, as Connecticut and New York have done, but he suggested that would not cure the problem.
The new cases, he said, do not appear linked to reopened schools or businesses, and are instead believed to be connected with private indoor gatherings that are harder to regulate.
Mr. Murphy and the state’s health commissioner, Judith M. Persichilli, warned that cases could continue to climb as the weather cooled and people shifted their activities indoors, where the risk of spread was higher.
“It is understandable that residents want life to go back to normal,” Ms. Persichilli said. “But as we approach a holiday season, now is the time to double down on social distancing, wearing face coverings and good hand hygiene.”
In New York, the rate of positive virus tests was 1.21 percent on Monday, compared with 3.36 percent in New Jersey.
Over the last seven days, Connecticut has seen an average of more than 375 cases per day, according to data collected by The Times, and a seven-day average positive test rate of 1.9 percent.
In response to higher rates of infection in parts of New York, Gov. Andrew M. Cuomo ordered shutdowns in certain neighborhoods in New York City and in parts of Rockland and Orange Counties.
Asked on Monday if New York intended to bar travelers from New Jersey, Mr. Cuomo said that was unlikely.
“You can’t do border patrol with New Jersey and Connecticut,” Mr. Cuomo said.
Even though many businesses are still asking employees to work from home, plenty of people still travel every day among the three states.
In Connecticut, 11 municipalities have been labeled “red” — meaning they had a two-week rolling average of 15 positive cases of coronavirus per 100,000 people. Towns and cities in the red zones must cancel public events and postpone indoor activities and outdoor activities where social-distancing or mask wearing is not possible.
“The state-by-state quarantine is complicated,” said Connecticut’s governor, Ned Lamont, noting the town-by-town differences within each state.
Weeks ago, New Jersey officials dispatched extra contact tracers to Ocean County on the Jersey Shore and set up a testing site at a baseball stadium to confront a large outbreak that was thought to have been linked to religious gatherings in Lakewood, N.J.
Cases there remain stubbornly high, but the virus is also now spreading throughout the state.
Mr. Murphy called the trend “sobering”: Five of the state’s 21 counties each reported more than 100 new cases on Monday. The seven-day average of new cases has not been this high since late May, and the number of people hospitalized with Covid-19 also is increasing slightly.
On Sunday, the state reported 1,282 new cases of the virus. On Monday, there were 1,192, including more than 100 each in Ocean, Essex, Union, Middlesex and Bergen Counties.
Still, Mr. Murphy continues to face pressure to reopen the state more fully.
He had suggested last week that he was preparing to relax the limits on indoor dining beyond the current 25 percent occupancy limits, offering a lifeline to restaurants struggling to stay afloat.
Most schools in the state are offering only partial in-person instruction, and classes in many districts remain entirely online. But last week, he said schools could move ahead with full-contact indoor winter sports like wrestling and basketball.
On Monday, he acknowledged the risk of sending “mixed messages” about reopening at a time when virus cases were on the rise.
“Things are on the table,” he said, referring to indoor dining, “but we’re looking at community spread.”
“I don’t want to take a step and have to lurch backward,” he added.
Amanda Rosa contributed reporting.