The State Department is advising U.S. citizens traveling in Europe to be aware of a “heightened risk of terrorist attacks” during the holiday season.

The advisory, issued Monday, says “credible information” indicates that Islamic State and al-Qaida militants continue to plan attacks in Europe around the holiday season. It also warns that “extremist sympathizers or self-radicalized extremists” may try to execute attacks.

The department is encouraging travelers to be cautious at holiday festivals and outdoor markets. It is also urging Americans to avoid large crowds and be vigilant when visiting tourist sites, riding public transportation and meeting in places of worship, hotels, restaurants and other gathering spots.

Other recommendations advise traveleres to:

--Follow the instructions of local authorities.  Monitor media and local information sources and factor updated information into personal travel plans and activities.

--Be prepared for additional security screening and unexpected disruptions.

--Stay in touch with family members and ensure they know how to reach you in the event of an emergency.

--Have an emergency plan of action ready.

The advisory praises European efforts to disrupt plots, but says the U.S. remains concerned about the potential for attacks. The alert is set to expire on Feb. 20, 2017.