Jewish community centres and schools in a dozen US states have reported bomb threats, days after hundreds of headstones were knocked over at a Jewish cemetery.

The Anti-Defamation League, a Jewish NGO, said there were at least 20 bomb threats against Jewish community centres and schools on Monday in 12 states across the eastern half of the country.

The group described the latest threats as the "fifth wave" observed in 2017. For the year, about 90 such threats have been received, including one against the league's New York headquarters.

No bombs were found after the latest threats.



"While this latest round of bomb threats to Jewish community centres and day schools across the country again appears to not be credible, we are nonetheless urging all Jewish institutions to review their procedures," the league's chief executive Jonathan Greenblatt said.

He described the bomb threats as "not the only manifestation of anti-Semitism in recent weeks," after widespread damage to Philadelphia's Mount Carmel Cemetery, where at least 75 headstones were toppled at the weekend.

The burial ground has been in use since the mid-1800s by the Pennsylvania city's Jewish community. A week earlier, more than 150 headstones were damaged at a Jewish cemetery in St Louis, Missouri.

READ MORE: Muslims raise $55,000 to fix vandalised Jewish cemetery

Pennsylvania Governor Tom Wolf vowed to protect the Jewish community and find the perpetrators of the cemetery desecration.

"Any anti-Semitic act or act of intimidation aimed at Jewish institutions and people in Pennsylvania is truly reprehensible," he said.

White House spokesman Sean Spicer said that President Donald Trump was "deeply disappointed and concerned" by the reported "cowardly destruction" in Philadelphia.

"The president continues to condemn these and any other form of anti-Semitic and hateful acts in the strongest terms," Spicer said.