BOSTON (CBS) – The State Department has issued a travel alert for Americans planning to visit Europe in wake of the terrorist attacks that killed at least 34 people Tuesday in Brussels.

The travel alert expires June 20.

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“U.S. citizens should exercise vigilance when in public places or using mass transportation,” the State Department said in its alert. “Be aware of immediate surroundings and avoid crowded places. Exercise particular caution during religious holidays and at large festivals or events.”

American travelers are asked to follow instructions from local authorities, monitor media reports, and register in the Smart Traveler Enrollment Program (STEP) before traveling.

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In addition, the State Department said travelers should expect additional security.

U.S. citizens are encouraged to stay in touch with family members during trips and make sure they have updated contact information.

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More information on the travel alert is available on the State Department’s website.