As the coronavirus surges in other parts of the United States, officials in Pennsylvania and New Jersey said Tuesday they were concerned about avoiding such an uptick in cases here, particularly warning residents not to allow reopenings to coax them into relaxing.

“This epidemic is definitely not over,” said Philadelphia Health Commissioner Thomas Farley. “The risk right now is rising.”

The positivity rate has increased in South Jersey and new infections and hospitalizations have increased in Delaware, which Gov. John Carney said Tuesday was “a little concerning.” His state also reported a significant jump in the number of deaths — now at 504 — after an audit discovered deaths previously undisclosed to the state.

More New Jerseyans 29 years old and younger are contracting the virus, now making up more than one-fifth of new infections. The increase is concerning health officials, who don’t believe it can solely be attributed to increased testing.

Though New Jersey’s overall statewide transmission rate remains good, Gov. Phil Murphy said it was beginning to creep up in several counties, though he did not identify which ones. He also said hospitalizations and the use of ventilators were slightly up in recent days.

“We can’t get our economy back to where it was if our hospitals fill back up with COVID-19 patients because some people mistakenly thought they were invincible,” Murphy said. “What we cannot have is a one-day increase in our health metrics becoming a trend because people gave up on social distancing.”

The warnings came as Dr. Anthony S. Fauci, the nation’s leading infectious-disease expert, told members of Congress that the next two weeks would be “critical” in addressing “disturbing” surges in nearly half the states. Speaking to a House committee Tuesday, he said the future looks like a “mixed bag” as the country remains in the first wave of the pandemic.

Meanwhile, at the city’s virtual news briefing, “there is mixed news about the epidemic today,” Farley said. “With cases surging in many parts of the United States, the impact on Philadelphia so far is unclear, but there are plenty of reasons to be concerned.”

Gov. Tom Wolf reiterated pleas for residents to wear masks, which he said is “a vital measure” to prevent spread as the entire state moves toward the “green” phase of reopening. It is required in all businesses in yellow and green counties.

“Pennsylvania has emerged as a leader among states for reduced cases amid increased reopening, and we want that to continue,” Wolf said in a statement.

At the same time, reopening plans continued.

Outdoor amusement and water parks in New Jersey can reopen July 2, including rides on boardwalks, Murphy announced Tuesday.

Attendance at amusement parks will be capped at 50% capacity. Murphy said that “specific guidance is currently being finalized and will be forthcoming” but that it is anticipated that employees and attendees will be required to wear masks “whenever practicable.”

“We are excited to welcome families back onto the piers again,” said Will and Jack Morey of Morey’s Piers in Wildwood. “We are working hard to redefine family fun this summer.”

Murphy also announced that playgrounds across the state will be allowed to reopen.

“Keep your distances. Wear your masks. Be smart and courteous — the world isn’t about you,” he said. “Don’t be the knucklehead who ruins it for everyone else.”

New Jersey has reached 169,734 confirmed positive cases, and 12,949 people have died. The number of deaths due to the coronavirus in the last three months is more than twice the number of New Jerseyans who have died of the flu over the last five years, Murphy said.

Pennsylvania has confirmed 82,696 cases of the virus, and 6,464 people have died, officials reported Tuesday. The commonwealth also reported 631 probable, not confirmed, cases.

Philadelphia is planning to move into the green phase of reopening by July 3, Farley said, but that move will depend on continuing to meet targets for decreasing cases and other metrics. Swimming pools and beauty salons will be able to open Friday.

He reminded residents that Philadelphia’s green phase will be more restrictive than in the rest of the state and repeated cautions that the city could postpone the move to green or “take steps back” if the spread of the virus changes.

“We have to watch this very closely, and that’s the real key message of today,” he said.

Philadelphia’s contact tracing program for new cases should be “fully operational” by July 1, Farley said, and daily case counts continue to decrease in the Pennsylvania counties surrounding the city. Philadelphia has hired 58 people to trace contacts and plans to hire 25 more by July 6.

About 5% of people tested in the last week in Philadelphia were positive for the virus, Farley said; the city is averaging about 100 new cases a day. The city continues to increase its testing capacity, now doing about 200 more tests per day in the last week than in late May. South Jersey also has about a 5% positivity rate, state health officials said Monday, higher than the other regions of the state.

Philadelphia’s Wawa Welcome America festival — set to be held virtually on Fourth of July weekend — will not include the traditional live fireworks display over the Art Museum this year, a spokesperson said Tuesday. Instead, a recording of last year’s fireworks show will be broadcast on NBC 10, Telemundo62, CoziTV, and TeleXitos.

Many cities and towns have canceled Independence Day events: Camden also canceled its annual Fourth of July waterfront festival, officials said Tuesday, and Ocean City will not have fireworks or its popular Night in Venice boat parade originally planned for July 18.

Meanwhile, reports of residents setting off fireworks in Philadelphia have increased substantially during the pandemic, Managing Director Brian Abernathy said Tuesday as he asked residents not to do so as July 4 approaches.

“The simple fact is that these are extremely dangerous products and the risk far outweighs the momentary excitement of explosions,” Abernathy said, citing accidents last year that injured children.

Philadelphia law prohibits setting off fireworks within 150 feet of occupied properties, and on public or private land without permission of the owner. Highly explosive fireworks, called class M, are banned in the city, as is selling fireworks without a license.

Abernathy urged residents who witness sales on the street or see firework explosions too close to occupied buildings to call 911 or their local police district.

Staff writers Rob Tornoe, Amy S. Rosenberg, and Oona Goodin-Smith contributed to this article.