Christmas 2016 Terror Threats: Berlin Market, Zurich, Turkey Attacks Part Of 'Day Of Terror'

Within the past 48 hours, Berlin, Zurich, Turkey, Yemen and Jordan have each seen terror arrive at their doorsteps, right in the heart of the international holiday season.

The U.K.'s terror threat level was raised to "severe" amid a spike in international terror attacks, including one targeting a Christmas market in Berlin on Monday evening. That attack followed a warning previously released by U.S. authorities advising travelers visiting markets across Europe over the holiday season remain vigilant.

As travel and congregating in public squares and shopping centers typically increases during the holiday season, so do global terror threats, with several already thwarted attempts in France and the United States in recent months. A similar, large-scale attack as the one that occurred this week in Berlin was prevented by French authorities, who detained seven suspects Nov. 21, the New York Times reported. Meanwhile, authorities in Kansas arrested four men planning an attack on an apartment complex housing a majority of Somali immigrants the month before.

Still, attacks have continued to go on around the world in recent days. Most recently, a 22-year-old off-duty Turkish police officer assassinated Russian ambassador to Turkey Andrey Karlov on Monday, an incident Russia, Turkey and U.S. President-elect Donald Trump all described as an act of terror. Suicide bombers in the southern Yemeni city of Aden killed at least 52 soldiers, the second attack the Islamic State claimed responsibility for in the country this month.

A gunman also opened fire on a prayer center frequently visited by Somali immigrants in Zurich, Switzerland’s largest city. He wounded at least three victims before reportedly committing suicide. At least ten people were killed and another 34 wounded on Sunday in Jordan, in what officials described to be a "cowardly terrorist attack." Gunmen in multiple locations opened fire on authorities before firing rounds at tourists in an ancient castle, killing a Canadian tourist, Jordanian civilians and at least seven security forces, CNN reported.

Looming threats and the variety at which terror groups are coordinating and inspiring attacks on civilians worldwide has created an unprecedented challenge for governments over the holiday season.

In the U.S., the terror threat remained at "elevated," warning of a significant risk of terror attacks on the homeland and citizens abroad. Experts said officials will be ramping up efforts over the holidays to continue preventing terror threats, in any form that may arrive.

"The intelligence agencies will be on high alert gathering as much info as is available both through computer communication, chatter on the waves of mobile phones, and through their sources on the ground, both in the Middle East and at the location of the incident," counterterrorism analysis Anthony Roman told ABC News.