A ‘swatting’ spree which saw hoaxers call armed police to 199 Jewish sites in the U.S. over the weekend was orchestrated from outside the country.

Assistant FBI Director Cathy Milhoan made the revelation in a confidential memo to law enforcement agencies.

"At this time, based on similar language and specific email tradecraft used, it appears the perpetrators of these threats are connected,” she wrote.

“Additionally, these threats appear to be originating from outside of the United States.

"To date, none of these email threats have involved any actual explosive devices or credible risk of harm to congregants."

More than 30 FBI field offices are investigating the threats.

A record number of swatting incidents and false bomb threats were made between Friday and Saturday, according to the Secure Community Network that advises U.S. Jewish institutions on safety.

It tracked 93 in California, 62 in Arizona, 15 in Connecticut, five in Colorado, and four in Washington state, according to a press release.

The network said that swatting incidents are up 540% this year. More than 449 swatting incidents and bomb threats have so far taken place.

Secure Community Network CEO Michael Masters said: “The alarming volume of swatting incidents and false bomb threats being carried out across the country is a major concern for the safety and security of the Jewish community in North America, as well as law enforcement.

“SCN, community partners, law enforcement, and public safety agencies take these incidents seriously and are working hard to address them.

'It's critical to recognize that these are not victimless crimes or innocent pranks: they can have real – and even deadly – consequences.”