Security has been stepped up at Jewish organisations in Chicago following the attempt to mail two bombs to synagogues in the city.
Jewish groups across the United States are on high alert after two packages containing explosive material hidden inside printer cartridges were intercepted on planes coming out of Dubai and East Midlands airports.
John Brennan, the US deputy national security adviser, has cautioned that there might be more attempts of the same type.
He said US authorities are investigating the likelihood of the bombs being “detonated en route to those synagogues, aboard the aircraft, as well as at the destinations,"
He added: “It would be very imprudent ... to presume that there are no others (packages) out there."
The terrorist organisation al-Qaeda in the Arabian Peninsula, which operates out of Yemen, is believed to be behind the attack.
One of the synagogues targeted is though to have been a Reform congregation serving a gay, lesbian, bisexual and transgender community.
The Jewish Federation of Metropolitan Chicago said it had alerted synagogues in the area not to open suspicious packages or any with “Yemen” in the address.
The Anti Defamation League called on Jewish communal institutions to “increase mailroom security and to contact law enforcement immediately if they see anything suspicious."
The statement said: “There is a reported threat to Jewish institutions in the form of packages mailed from overseas, particularly Great Britain, Yemen and Saudi Arabia."
The Community Security Trust (CST) said while intelligence had not suggested the plot was intended to target the UK Jewish community, “we must be mindful of all possibilities.”
They said: “This emphasises yet again the enduring terrorist threat against Jewish communities and the need to maintain a high state of alert.
“CST recommends that those responsible for accepting the delivery of post and parcels at Jewish locations must be alert for suspicious items, especially unexpected post and deliveries.”