The alert level was elevated to Force Protection Condition (FPCON) Charlie at bases including the US Army garrison in Stuttgart, Germany, where US European Command (USEUCOM) is headquartered, CNN said on Sunday, Anadolu reported, citing two US officials.

One US official stationed in Europe said that such a high alert level has not been seen in at least a decade, indicating an "active-reliable threat," the report said.

USEUCOM spokesperson Commander Dan Day confirmed that USEUCOM continuously assesses various factors affecting the safety of US military personnel abroad and remains vigilant.

The exact intelligence that triggered the increased security measures remains unclear. However, European authorities have recently heightened security due to potential terror threats, particularly in anticipation of the Paris Olympics in July and amid the ongoing European football championships in Germany.

In response to the threat, the German government has deployed 580 international police officers to support local security efforts.

France has also heightened its national security alert system to its highest level since March, in preparation for the Olympic Games.