The caller - speaking German, possibly with a Russian or Turkish accent - phoned a publicly listed number at the Spangdahlem Air Base on Monday evening, the day before Tuesday's sixth anniversary of the Sept. 11 attacks, police in nearby Trier said.
He threatened to attack the base with at least four accomplices, and the word "bombs" was mentioned, according to a police statement.
The U.S. military informed German police, who took "immediate measures to protect the air base," the statement said.
Police spokesman Reinhard Rothgerber said those measures were still in place Tuesday. "That means that the protection of this facility, which is at a high level anyway, was intensified further," he said.
Later Tuesday, Rothgerber said that "according to our assessment, there was no concrete danger to the facility or the population."
He said police were still trying to track down the anonymous caller.
A Spangdahlem base spokeswoman confirmed that a telephone threat against the base was received on Monday.
"We have no additional information to validate a direct threat against Spangdahlem; however, we take every threat seriously," said the spokeswoman, Staff Sgt. Tammie Moore. "The security of the base and its personnel, both military and civilian, is critical, and we are taking measures to ensure they are protected."
Germany is particularly sensitive to possible terrorist threats after three men were arrested last week on suspicion of planning massive bomb attacks on U.S. and other facilities.
The U.S. military's Stuttgart, Germany-based European Command, or EUCOM, said the Spangdahlem threat had no impact on its overall security measures.
"U.S. forces remain at a heightened state of alert, but there have been no EUCOM-wide changes in our force protection conditions based on the incident in Spangdahlem," spokesman Maj. John Dorrian said.
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The TurkStream, which runs along the bottom of the Black Sea from Russia's Anapa to Turkey, will consist of two lines, each with a capacity of 15.75 billion cubic meters of gas a year