US bases located across Europe raised the “Charlie” alert level, ABC News and CNN reported, citing unnamed officials.
That level is ordered when an incident occurs or intelligence is received indicating the likelihood of some form of terrorist action or targeting of personnel or facilities, the US military said on its website.
The highest level “Delta” is applied when a terrorist attack has occurred or is “imminent”.
US European Command (USEUCOM) did not confirm the status change when contacted by AFP, but said: “we remain vigilant.”
The Pentagon, meanwhile, says that due to a combination of factors that potentially affect the safety of U.S. service members and their families stationed in Europe, U.S. European Command is redoubling its efforts to emphasize caution during the summer months.
The State Department is currently advising American citizens in Germany, where USEUCOM is headquartered, to be on alert for terrorism.
Although no specific threat was mentioned, European nations have been on heightened alert since gunmen killed nearly 150 people outside Moscow in March, in an attack claimed by the Islamic State group (ISIL).
France has also increased its alertness ahead of the Paris Olympics, while Germany is currently hosting an international soccer tournament.
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