The United States has called on its citizens to exercise "additional vigilance" when visiting popular tourists attractions in Europe during the upcoming holiday season.

In a travel alert issued Thursday, the US State Department said a series of terror attacks across Europe showed that international terror groups such as the "Islamic State" and al-Qaida were still capable of executing attacks on the continent.

Washington has been warning its citizens to beware of extremist attacks in European cities, especially during festive events. The latest travel alert, which specifically mentions the holiday season, expires on January 31, 2018. 

Read more: Should I go? The impact of government advisories for travelers to Europe

The alert mentions deadly attacks at a Christmas market in the German capital Berlin in December and a nightclub in Istanbul in Turkey on New Year's Eve. 

"US citizens should always be alert to the possibility that terrorist sympathizers or self-radicalized extremists may conduct attacks with little or no warning," the State Department warned.

The alert said extremists continued to focus on tourist locations, transportation hubs, markets, shopping malls, and local government facilities as viable targets. It also listed a host of possible soft targets like airports and restaurants.

The State Department said it continues to work closely with its European partners and allies on the threat from international terrorism and asked those traveling between countries in Europe to check the local US embassy or consulate website for any recent security messages.