Jewish centres and schools across the nation coped with another wave of bomb threats Monday as officials in Philadelphia made plans to repair and restore hundreds of vandalized headstones at a Jewish cemetery.

Jewish Community Centers and day schools in at least a dozen states received threats, according to the JCC Association of North America. No bombs were found. All 21 buildings — 13 community centres and eight schools — were cleared by Monday afternoon and had resumed normal operations, the association said.

The threats, all of which appeared to be hoaxes, were received in Alabama, Delaware, Florida, Indiana, Maryland, Michigan, New Jersey, New York, North Carolina, Pennsylvania, Rhode Island and Virginia. For some centres, it was the second or third time this year they had been targeted.

Police in Mercer Island, Wash., also reported a community centre was targeted by a bomb threat.

"Members of our community must see swift and concerted action from federal officials to identify and capture the perpetrator or perpetrators who are trying to instill anxiety and fear in our communities," David Posner, a director at the JCC Association, said in a statement.

Jewish Cemetery Damage

People gather in prayer amid the toppled headstones and damage at Mount Carmel Cemetery on Monday in Philadelphia. (David Swanson/The Philadelphia Inquirer via AP)

The San Francisco office of the Anti-Defamation League, a watchdog group that monitors hate groups, received a bomb threat and was evacuated while police investigated, the Anti-Defamation League said in a statement. Jewish groups, President Donald Trump and Israeli officials have condemned the surge in disruptive intimidation, as well as the vandalism of Jewish cemeteries.

White House spokesman Sean Spicer raised the subject at a news briefing on Monday. "The president continues to condemn these and any other forms of anti-Semitic and hateful acts in the strongest terms," he told reporters, saying they were in breach of the country's founding principles.

The FBI and the Justice Department's Civil Rights Division are probing the threats.

Attorney General Jeff Sessions called the vandalism and bomb threats serious, unacceptable behaviour and said the department will "do what it can to assist in pushing back ... and prosecuting anybody that we can prove to be a part of it."

"We are a nation that is a diverse constituency, and we don't need these kind of activities," Sessions said.

Companies, Muslim groups come to aid

In Philadelphia, police investigated what they called an "abominable crime" after several hundred headstones were damaged during the weekend at Mount Carmel Cemetery, a Jewish cemetery dating to the late 1800s, said Steven Rosenberg, chief marketing officer of the Jewish Federation of Greater Philadelphia.

Jewish Cemetery Damage

A security guard stands outside the entrance to the David Posnack Jewish Community Center and David Posnack Jewish Day School after people were evacuated because of a bomb threat Monday in Davie, Fla. (Wilfredo Lee/The Associated Press)

Police said the vandalism appeared to be targeted at the Jewish community, though they cautioned they had not confirmed the motive. Philadelphia Mayor Jim Kenney said authorities were doing everything possible to find those "who desecrated this final resting place."

"I'm hoping it was maybe just some drunk kids," said Aaron Mallin, who discovered the damage during a visit to his father's grave. "But the fact that there's so many, it leads one to think it could have been targeted," he told WPVI-TV.

The vandalism comes less than a week after a Jewish cemetery in suburban St. Louis was targeted.

Both acts of vandalism spurred offers of help.

The Philadelphia Building & Construction Trades Council, an umbrella group for more than 50 union locals that work in the construction industry, offered to repair the damage at Mount Carmel free of charge, calling it a "cowardly act of anti-Semitism that cannot be tolerated." A community cleanup organized by the Jewish Federation was to begin Tuesday with as many as 50 volunteers per hour.

And in Missouri, a Muslim crowdfunding effort to support the vandalized Jewish cemetery near St. Louis had raised more than $136,000 by Monday, with organizers announcing they would use some of the money for the Philadelphia cemetery.

With files from Reuters