Three people have been arrested in Germany on suspicion of planning attacks on Frankfurt airport, one of Europe's busiest, and a US military base.
The German federal prosector's office said they were suspected of having links with an "Islamic terrorist organisation".
Two of those arrested were German nationals and the third was a Pakistani passport holder, public network radio, Suedwestrundfunk (SWR), reported on Wednesday.
Franz Josef Jung, the country's defence minister, said there had been "an imminent threat," but did not explain his remark any further.
"The security services have done a very good job and I can't say anything more at this point," he said.
Targets
Frankfurt international airport is one of Europe's busiest and the Ramstein base in the nearby state of Rhineland-Palatinate, 130km southwest of the airport, is one of the most important US air bases overseas.
Ramstein is a crucial transit point for US military operations in Iraq and Afghanistan.
Germany, which has forces stationed in Afghanistan, has been on high alert for attacks.
The country has feared a re-emergence of Islamic groups since 2001, when the northern port city of Hamburg was used as a base for the September 11 attacks in the United States.
Earlier this year, federal prosecutors charged a Lebanese man held in detention over an unsuccessful attempt to detonate bombs on two trains in Germany in 2006.