WASHINGTON - 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 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.

The advisory praises European efforts to disrupt plots, but says the U.S. remains concerned about the potential for attacks.