A man called a U.S. air base in western Germany and threatened to attack it, prompting police to step up security measures, police said Tuesday.

The man — speaking German, possibly with a Russian or Turkish accent — called a publicly listed number at the Spangdahlem 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 immediately 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.

He said police were trying to track down the anonymous caller.

A 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.