On Sept. 9, Interpol, the international police organization, warned of a possible terrorist attack targeting Europe.
French officials said they have received credible information of what they say is "a real threat."
The warning says terrorists may be targeting tourists in commando-style raids like the deadly 2008 Mumbai attacks in India.
The U.S. Department of State is advising Americans visiting or traveling in Europe to be cautious.
In September, France raised its security level at tourist sites like Notre Dame Cathedral and the Eiffel Tower.
French authorities recorded nine bomb alerts in Paris in September, including two at the Eiffel Tower - a threefold increase from a year earlier. No explosives were found.
Some frequent fliers at Los Angeles International Airport said they would not cancel their plans, but they will be more cautious.
"Pay attention, and it happens when it happens," said traveler Frauke Kuo. "When we were living in Germany 40 years ago, it was the same thing. Nothing has changed. It's just that everybody is more aware these days."