Officials have urged congregations to take security precautions.

The Federal Bureau of Investigation is warning synagogues in New Jersey of a broad security threat after getting what it called “credible information” about an increased level of risk.

“We ask at this time that you take all security precautions to protect your community and facility,” the F.B.I.’s Newark office wrote on Twitter

“We will share more information as soon as we can. Stay alert. In case of emergency call police.”

The warning comes as incidents of antisemitic bias and violence have been on the rise across the country.

Nationwide last year, there were 525 known incidents of harassment, vandalism and assault at Jewish institutions, including synagogues, community centers and schools — an increase of 61 percent from 2020, according to an audit by the Anti-Defamation League.

Several recent bias crimes have generated headlines across the world. In January, a British national took four people hostage at a synagogue in suburban Fort Worth, Texas, for 11 hours, before being killed by the authorities. And in 2018, a man armed with an AR-15-style assault rifle and several handguns showed up at Shabbat services at the Tree of Life synagogue in Pittsburgh and killed 11 worshipers.

A spokeswoman for the F.B.I. in Newark was not immediately available for additional comment about the nature of the threat in New Jersey. But as a result of the warning, the New York Police Department immediately announced it had stepped up its monitoring to ensure the safety of “every area that encompasses our Jewish citizens and synagogues.”

Rabbi David Levy, director of the American Jewish Committee’s New Jersey office, described the urgency of the F.B.I. warning as rare.

“It is a reflection of rising antisemitism in our country and how that antisemitism can morph into violence,” Rabbi Levy said, adding, “Everybody is taking this seriously.”

Gov. Philip D. Murphy of New Jersey said the attorney general’s office and the State Office of Homeland Security and Preparedness were “working with local law enforcement to ensure that all houses of worship are protected.”

Jason M. Shames, chief executive of the Jewish Federation of Northern New Jersey, said his organization immediately activated its community security warning system. Threats tied to individual congregations were not uncommon, he said, but it is “rather unusual” for the warning to come directly from the F.B.I.

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“Which seems to indicate to me that there’s something very unique about this,” Mr. Shames said.

In Lakewood, N.J., home to a large community of Orthodox Jewish families, police officers were asked to remain on duty for a second shift. One large synagogue in the township notified residents it would remain locked through the day and until further notice as a result of the warning.

Representatives of the Lakewood Police Department were participating in a conference call with the F.B.I., said Raymond G. Coles, the township’s mayor.

“We are looking into additional resources should the situation warrant it,” Mr. Coles said.

Osniel Rozen, 29, of Lakewood, said he would be “extra careful,” but had no plans to alter his routines.

“I don’t think, though, that anyone will be shutting down their daily life because of this tweet from the F.B.I., unless there is some more specific information,” he said.

In Bloomfield, N.J., Rabbi Marc Katz, of Temple Ner Tamid, said this latest threat only added to a growing sense of anxiety among his congregation of 540 families.

Over the past several years, the temple has added boulders out front as blockades, layered a shatterproof film on its windows and replaced locks. A armed guard is on duty every time the school or synagogue is in use, and there are cameras everywhere, he said.

He said he expected that children enrolled in the synagogue’s preschool program may be kept from going outside on Fridays. But long before the F.B.I. warning, he said, a sense of heightened alert has been the rule, not the exception.

“We’re doing a lot right now, so any changes that we’re making are going to be very minor,” he said.

State Assemblyman Gary S. Schaer, who represents Passaic County and is considered a strong ally of Jewish causes, said he could not recall a similar F.B.I. alert in his 16 years in the State House.

Representative Josh Gottheimer said he had arranged a call with every synagogue in his congressional district and law enforcement officials to brief Jewish leaders on steps that are being taken to increase police patrols and security.

“These additional terroristic threats against the Jewish community are alarming, to say the least,” said Mr. Gottheimer, a Democrat whose district includes Bergen County, N.J., which has a large Jewish population.

“The key is to stand up and fight it, and not back down, and make it very clear that we will not cower.”

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