Germany has raised its security alert at two air force bases after receiving indications of possible terrorist attacks, a defense ministry spokesman said on Tuesday. The bases include the air training center at Rhein and the fighter-bomber squadron at Büchel -- which has around 1,700 troops. Both are located near Germany's western border. The alert status at both had been raised to "Charlie", the second-highest of four security levels in the German armed forces. The heightened security status will be in force this week and the last week of May. The spokesman refused to give any details of the origin of the possible attacks but added that the heightened security alert meant increased checks at the entrance to the bases and tighter protection. "It's a purely precautionary measure," he said.