Two tourists look a map Sunday in Madrid. The State Department has cautioned Americans traveling in Europe to be vigilant because of heightened concerns about a potential al-Qaida terrorist attack aimed at U.S. citizens and Europeans.

Madrid ? A rare advisory for U.S. travelers to beware of potential terrorist threats in Europe drew American shrugs Sunday from Paris to Rome, but tourism officials worried that it could deter would-be visitors from moving ahead with plans to cross the Atlantic.

The travel alert is a step below a formal warning not to visit Europe, but some experts said it could still hurt a fragile European economy already hit hard by the debt crisis.

“I think if someone was looking for an excuse not to travel, then this is just the ticket,” said George Hobica, founder of Airfarewatchdog.com. “However, I don’t think most people will alter their plans unless the threat is very specific.”

The State Department alert advised the hundreds of thousands of U.S. citizens living or traveling in Europe to take more precautions about their personal security. Security officials say terrorists may be plotting attacks in Europe with assault weapons on public places, similar to the deadly 2008 shooting spree in Mumbai, India.

Without a specific threat, however, American visitors were not letting the alert disrupt their travels.

“We live in New York. So in New York we think about these things all the time,” said Richard Mintzer, a 55-year-old American visiting Italy with his wife. “I wouldn’t say we are particularly worried in Rome, no more than we would be at home, or anywhere in the Western world.”

At Paris’ spring-summer 2011 ready-to-wear fashion shows, W magazine fashion market director Karla Martinez said she gets “worried for five minutes, but then I forget about it and get back to the job that I’m here to do.

“It’s a little scary when you’re staying in a big hotel with lots of tourists, because we hear that could be a target, but I try not to get too worked up about it,” she said. “At the end of the day all you can do is keep your eyes and ears open and try not to be naive.”

The nonprofit group IES Abroad sent e-mails Sunday warning about 1,500 college students in its European study abroad programs to avoid crowded tourist spots and hangouts typically frequented by Americans. The message — also sent to the students’ parents — also told students to leave public places if they see signs of trouble.

“We say, ‘Be alert, cautious and aware of your surroundings,”‘ IES executive vice president Bill Hoye said. “It means, ‘Don’t be totally plugged into your iPod.”‘

Hours after the e-mails were sent by the Chicago-based group, it had no sign of any students who wanted to drop out of the programs.

The impact on travel could deepen if the threat leads to new, tighter security measures, said Henry Harteveldt, a travel analyst for Forrester Research. But the U.S.-based Air Transport Association, a trade group for the airline industry, said it expects “business as usual.”

United, Continental and Delta said they were operating as usual on Sunday without any cancellations or delays related to the terror alert. The airlines said customers will be charged the usual penalty if they want to change itineraries.

Kevin Mitchell, chairman of the Business Travel Coalition, said business travelers will likely keep their plans and hold onto nonrefundable tickets as long as the warning remains “fairly general.”