US airports and mass transit systems will tighten security in response to apparent terrorist incidents in Britain, the Bush administration said. The United States, however, is not raising its terror alert status, President George W. Bush's spokesman and the Homeland Security secretary said Saturday. "There is no indication of any specific or credible threat to the United States - no change in the overall security level," Tony Snow told reporters in Maine. Britain raised its security alert to the highest level possible, an indication that terrorist attacks are imminent. Snow said the British government had notified the White House in advance, and that the move did not provoke any change in the threat assessment in the United States. "We constantly monitor and assess the situation, and adjust our posture as necessary," Snow said.