The American bases, located across Europe, were raised to the "Charlie" alert level, ABC News and CNN reported, both citing unnamed officials.

That level is ordered when "an incident occurs or intelligence is received indicating some form of terrorist action or targeting against personnel or facilities is likely," the US Army says on its website.

The highest level "Delta" is applied when a terrorist attack has occurred or one is "imminent."

The US European Command (USEUCOM) did not confirm the status change when contacted by AFP, but said: "we remain vigilant."

The Pentagon meanwhile said that "due to a combination of factors potentially impacting the safety and security of US service members and their families stationed in the European theater, US European Command is redoubling its efforts to stress vigilance during the summer months."

The US State Department currently advises American citizens in Germany, where the USEUCOM is headquartered, to exercise increased caution due to terrorism.

While no specific threat has been mentioned, European nations have been on heightened alert since gunmen in March killed nearly 150 people outside Moscow, an attack claimed by the Islamic State (IS) group.

France has also increased its state of alert ahead of the Paris Olympics, while Germany is currently hosting an international football tournament.