Germany's defense minister said Wednesday that three men have been arrested on suspicion of plotting attacks on Frankfurt's international airport and the nearby US military base in Ramstein "There was an imminent threat," Franz Josef Jung told Germany's ARD broadcaster. He declined to elaborate, but the Sudwestrundfunk public broadcaster said two of the suspects had German citizenship and the third was Pakistani. Sudwestrundfunk said the men were arrested Tuesday evening and were close to carrying out the attacks. German and US officials have warned of the possibility of a terrorist attack, and security measures have been increased. Germany, which did not send troops to Iraq, has largely been spared terrorist attacks like the train and subway bombings in Madrid and London - although its involvement in the attempt to stabilize Afghanistan against Islamic insurgents has led to fears it might be targeted. In July 2006, two gas bombs were placed on commuter trains but did not explode. Officials said that attack was motivated by anger over cartoons portraying the Prophet Muhammad in a Danish newspaper. Several suspects are on trial in Lebanon, and a Lebanese man has been charged in Germany. Frankfurt airport is continental Europe's busiest, and the air base at Ramstein is a major transport hub for US military operations in Iraq and Afghanistan.