The U.S. State Department issued a rare worldwide travel alert, warning of the widespread threats posed by members of the Islamic State or copycat bombers.
The alert said that there were increased terror threats from al-Qaeda, Boko Haram and ISIL following the Paris terror attacks.
The warning also said people unaffiliated with the groups may be inspired by recent terrorist attacks in France, Nigeria, Denmark, Turkey and Mali.
Millions of Americans are currently preparing to travel for the Thanksgiving holiday.
USA Today reports that organisers of major events like the Macy's Thanksgiving Day Parade are bolstering their security preparations.
"U.S. citizens should exercise vigilance when in public places or using transportation," the alert said.
"Be aware of immediate surroundings and avoid large crowds or crowded places. Exercise particular caution during the holiday season and at holiday festivals or events."
The warning is unusual in that is global, meaning American passengers are being asked to be on alert for possible terrorist attacks wherever they are traveling.
The last time the State Department issued a similar worldwide alert was Dec. 2014, USA Today reported.
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