Jewish community centres across the US have faced two waves of bomb threats this month, prompting an FBI investigation and a security advisory.

As reported by the JJC Association of North America, a total of 27 centers across 17 states were targeted this week, similar to threats faced by 16 centres in nine states last week, suggesting the scale of the threats has increased.

In both cases, there were evacuations and the police worked quickly to give the centres the all-clear. No actual bombs were found.

The FBI is investigating "possible civil rights violations in connection with threats".

David Posner of the JCC association said in a statement that the group is "concerned about the anti-Semitism behind these threats."

"While the bombs in question are hoaxes," he said, "the calls are not."

The Anti-Defamation League issued a security advisory to Jewish institutions across the country, adding that it was closely monitoring the situation and has been in touch with state and federal police.

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"Although so far these threats do not appear to be credible, we are recommending that Jewish communal institutions review their security procedures and remain in close contact with law enforcement," said ADL CEO Jonathan Greenblatt in a statement.

"While each incident needs to be taken seriously and investigated closely, thus far we are not aware of any of these threats being substantiated."

It was not clear whether the threats this week were connected with those of the previous week, but Paul Goldenberg, national director of the Secure Community Network, a nonprofit that advises Jewish groups on security, told Reuters that the number of threats was unheard of.

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The threats come shortly after a group of Neo-Nazis planned and failed to march in the small town of Whitefish, Montana, the hometown of white supremacist Richard Spencer.

The plans were prompted by an article in fascist website The Daily Stormer, in which Andrew Anglin picked up on comments made by Spencer’s mother, who blamed a non-profit called Loves Lives Here for blaming her for her son’s views. The organisation denied the allegations.

The discriminatory article was posted under a section of the website called "The Jewish Problem", posting contact details and pictures of people from the organisation, including a young boy, with yellow stars added to their clothing.

Fascists were halted, however, when the group failed to get a permit on time for the demonstration.

Neither the demonstrations planned for Montana nor the Jewish bomb threats were directly linked to Donald Trump.