Incidents in France, Russia, Sweden and the United Kingdom have prompted the U.S Department of State to issue a travel alert Monday.
The department said the Islamic State group, al-Qaida and their affiliates retain the "ability and to plan and execute" attacks in Europe.
The alert also said citizens should be especially vigilant at large, high profile events, particularly during the summer travel season.
Travel alerts are usually related to a short-term event. This includes political unrest, strikes or violence by terrorists.
The last terror alert was for Europe was issued in November 2016.
The warning issued Monday expires September 1, 2017.
The Associated Press contributed to this report.
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