The U.S. State Department has issued a travel alert for what it called the "continued threat of terrorist attacks throughout Europe."
The alert, which expires September 1, cites recent incidents in France, Russia, Sweden and Britain that the State Department says show the ability of Islamic state, al-Qaida or others to carry out attacks.
"U.S. citizens should always be alert to the possibility that terrorist sympathizers or self-radicalized extremists may conduct attacks with little or no warning," the alert says.
It further states that extremists focus on tourist areas, transportation hubs, markets and local government facilities.
The State Department issues travel alerts for short-term events it thinks travelers should know about.An alert is less severe than a travel warning, which the department issues to direct people to "consider very carefully" whether they should visit a country at all.