WASHINGTON, May 1 (Xinhua) -- The U.S. State Department on Monday issued a travel alert to U.S. citizens concerning continued threat of terrorist attacks throughout Europe.

In the alert, the State Department cited recent incidents in France, Russia, Sweden and the United Kingdom to demonstrate that extremist groups, including Islamic State, Al-Qaeda and their affiliates, "have the ability to plan and execute terrorist attacks in Europe."

"Extremists continue to focus on tourist locations, transportation hubs, markets/shopping malls, and local government facilities as viable targets," said the alert, adding that other public venues, such as hotels, restaurants and high-profile events could also be the targets.

"Terrorists persist in employing a variety of tactics, including firearms, explosives, using vehicles as ramming devices, and sharp-edged weapons that are difficult to detect prior to an attack," said the alert, which is valid till Sep. 1, 2017.

State Department's last travel alert for Europe was issued last November after French authorities foiled a terror attack plot ahead of the winter holiday season. The old travel alert expired this February.