The US State Department is warning Americans visiting Europe this summer about the potential for terrorist attacks.

The travel alert says major sporting events, tourist sites, restaurants and shopping centres are possible targets.

France is hosting soccer's European Championship and cycling's Tour de France, while under an extended state of emergency. Two-and-a-half million visitors are expected in Krakow, Poland, for the Catholic Church's World Youth Day in late July.

The department tells US citizens to be vigilant in public places or when using mass transportation. They also should be prepared for additional security screening and unexpected disruptions.

The warning expires on 31 August.