The previous alert, issued in winter, expired in February.
A State Department official reported the fresh alert wasn't based on a specific threat, but rather on a continuing threat of attacks.
The notification expires on September 1.
US issues travel alert for Europe
Alert instructs cautiousness in public spots, constant alertness
"US citizens should be alert to the possibility that terrorist sympathizers or self-radicalized extremists may conduct attacks with little or no warning," it warned.
It also advised caution at public spots like malls, hotels, clubs, restaurants and more.
Dos and don'ts for tourists
The alert urged tourists to monitor media and consider updates while making plans. It also asked people to "be prepared for additional security screening and unexpected disruptions". It recommended having an emergency plan ready. Travelers should register in the Smart Traveler Enrollment Program, it said.