American passport and Visa
American passport and Visa Thinkstock

The US State Department issued a travel advisory to American citizens planning on traveling to Europe, warning that the large crowds are likely to become a target for terror attacks. 

“We are alerting US citizens to the risk of potential terrorist attacks throughout Europe, targeting major events, tourist sites, restaurants, commercial centers and transportation,” it said in a statement. 

The release specifically mentioned the Catholic Church’s World Youth Day event, being held in late July in Krakow, Poland. 

"U.S. citizens should be aware that local infrastructure may be strained due to the large number of visitors," it warned. 

The European Soccer Championship in France, being held in June and July, is also a likely target. The Brussels bomber cell which bombed the Belgian capital's airport had, in fact, originally chosen it to be their target  - but changed their minds after police began closing in on their location.

The State Department urges citizens to "exercise vigilance" in large crowds and be alert in public places, keep updated with local media, and stay in touch with friends and family members throughout their trips. 

The travel alert expires on August 31.