The coronavirus is spreading in New Jersey at a higher rate than it has for months, Gov. Phil Murphy said Friday as he reported 699 new cases, the most for one day in over a month.

“The numbers are setting off alarms that we are taking very seriously,” Murphy said. “We are standing at a very dangerous place. ... The only way to silence these alarms and get back to the process of moving forward is for everyone to take them seriously. Not just most of you, but all of you.”

As hot spots rage around the country, New Jersey and Pennsylvania are attempting to keep case counts from rising further, hoping to stave off a full resurgence of the virus. Murphy said he could impose new restrictions on gatherings as soon as next week if needed, and Pennsylvania announced plans Friday to hire 1,000 more contact tracers to help monitor the spread of COVID-19.

The commonwealth reported 970 new cases on Friday, fairly close to the week’s seven-day average.

“We know the virus has not gone away as we see cases rise, so we must work together to stop another surge,” Health Secretary Rachel Levine said in a statement. Officials have said wearing masks, practicing social distancing, and following other safety protocols can stop it.

Daily new cases were rising in Mississippi, Georgia, Missouri, and elsewhere, while some of the recently hard-hit states, including Florida and California, saw small declines in new cases this week.

Anthony Fauci, the nation’s top infectious disease expert, told a House panel Friday morning that he is “cautiously optimistic” that a safe coronavirus vaccine will be available this year.

“We hope that as the time we get into the late fall and early winter, we will have, in fact, a vaccine that we can say would be safe and effective,” Fauci said. “One can never guarantee the safety or effectiveness unless you do the trial, but we are cautiously optimistic.”

And the federal program providing nearly 30 million American workers $600 a week in unemployment benefits ended Friday without an agreement on Capitol Hill about how to extend it.

Officials in the region remained concerned about young people spreading the virus at parties and other gatherings. In New Jersey, after days of warnings about cases traced to house parties, Murphy indicated the state would need to have a “really good” weekend and start of next week to avoid further restrictions on indoor gatherings.

And Airbnb announced it was cracking down on party houses in the Garden State, suspending or removing 35 listings, including along the Shore.

“We support [the governor’s] call to action to stop parties and promote behavior that respects the public’s health,” said Chris Lehane, Airbnb’s senior vice president of global policy and communications.

People 18 to 29 made up between a quarter and a third of New Jersey’s cases over the first three weeks of July, compared with 12% in April, Health Commissioner Judith Persichilli said this week.

The share of infections in young people have also jumped in every region of Pennsylvania. In the southeast, people 19 to 24 accounted for less than 5% of cases in April and approximately 19% of cases in July. In the southwest, the jump was nearly the same.

Pennsylvania and Philadelphia health officials have said in recent weeks that although death rates remain low, the case surges among young people — on Tuesday, for instance, more than half of new cases in Philadelphia were people under 40 — are a warning sign that they could rise again as the virus continues to spread.

Deborah Birx, the White House coronavirus response coordinator, warned about the phenomenon on Sunday while speaking in Kentucky, which this week reimposed restrictions after seeing case surges.

“Just to be very clear, what happens first with this new movement of cases is, young people are often infected first, particularly in the under-30 age group. They go on to infect — unknowingly; most of them are asymptomatic — they go on to infect their parents, who then infect or they infect the grandparents,” Birx said. “[That’s] what we’re trying to prevent.”

In the states that experienced major outbreaks in the last eight weeks, many had low death rates at the start of the surges that later rose. Florida’s seven-day average number of deaths per day quadrupled over the course of July; Arizona’s more than doubled; Georgia’s nearly quadrupled. Several of those states broke their records for the numbers of daily deaths this week.

Pennsylvania had confirmed 7,189 deaths as of Friday, and New Jersey had 15,809. Pennsylvania’s seven-day moving average for daily reported deaths was 14 on Thursday and New Jersey’s was 15, continuing a slow but largely steady decline that begin in May.

In Burlington County, reportedly identified by the federal government this week as a hot spot for the virus, the daily average of new cases has increased from about 15 to 35 new cases per day, county Health Director Herb Conaway said Friday.

“While this uptick is certainly cause for concern, we also believe that commonsense measures and adherence to the state’s social-distancing rules and guidance can contain the spread. This is becoming even more important with schools starting in just a few weeks,” Conaway said.

New Jersey’s transmission rate, which has hovered just under or just above 1.0 in recent weeks, hit 1.35, Murphy said, meaning every virus carrier infects 1.35 others.

Philadelphia officials on Friday reported 141 new infections. Health Commissioner Thomas Farley said this week that new cases of the virus are increasing and predicted that the pandemic will get worse before it gets better in Philadelphia.

In the week that ended last Saturday, the city had a daily average of 165 new cases.

After five days of isolation in Philadelphia because of a coronavirus outbreak, the infected members of the Miami Marlins will return to Florida by bus, the city confirmed Friday. Eighteen players and two coaches have tested positive, and they will return while the rest of the team waits to find out where the next game will be played.

The Phillies reported no new infections among the team and staff on Friday based on the previous day’s testing. City officials said contact tracers had not identified any new cases in the city that “have been positively linked to the Marlins.”

Staff writers Erin McCarthy, Matt Breen, Robert Moran, and Laura McCrystal contributed to this article.