US military bases across Europe, including Germany, were put on high alert over the weekend due to concerns of a potential terrorist attack.
US broadcaster CNN reported that the Pentagon had declared the second-highest security level at the US bases, citing two unnamed government officials.
This security level, known as Force Protection Charlie, applies when the Pentagon has received credible intelligence of a potential terrorist attack.
"There is credible intel pointing to an attack against US bases over the next week or so," US broadcaster Fox News quoted a defence official as saying. The Pentagon did not initially respond to a request for comment.
The nature of the alleged threat remained unclear. The official cited by Fox News said it did not relate to the parliamentary elections in France.
The United States European Command (EUCOM) headquarters in Stuttgart declined to comment on the specific protective measures taken by the armed forces when asked by CNN.
However, a spokesman stated that EUCOM regularly reviews "a variety of factors that play into the safety of US military community abroad."
"As part of that effort, we often times take additional steps to ensure the safety of our service members," the spokesman added, without providing any further details.