WASHINGTON -- US law enforcement has warned Jewish groups to be on the lookout for packages mailed from overseas in the wake of suspicious packages addressed to Jewish institutions in Chicago.The Associated Press quoted President Obama as saying Friday that the packages found on planes headed for the United States constituted a "credible terrorist threat" and had been addressed to Jewish organizations in the Chicago area.RELATED:The US is fighting a blind man’s war on terrorInternational Terrorism: Terrorism 2.0The Anti-Defamation League issued a nationwide alert, based on warnings from law enforcement officials."There is a reported threat to Jewish institutions in the form of packages mailed from overseas, particularly Great Britain, Yemen and Saudi Arabia," it said. "In light of the reported threat, the League has sent out a notice to US Jewish communal institutions across the country to increase mailroom security and to contact law enforcement immediately if they see anything suspicious."Queries should be directed to the FBI, the alert said. The Jewish United Fund/Jewish Federation of Metropolitan Chicago also issued an alert. "The Jewish Federation of Metropolitan Chicago has alerted area synagogues not to open packages that appear suspicious due to the way they are wrapped, or which come from Yemen or from any organization that has the name 'Yemen' in it," it said. "Any institution that receives such a package is advised to call “911” and to evacuate the premises until law enforcement arrives."