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. Still, US officials were wary. Acting out of "an abundance of caution" ahead of the US Independence Day holiday on July 4, Homeland Security chief Michael Chertoff said the government is putting in place plans to increase security at airports, on mass transit and at transportation facilities. "Some of these measures will be visible; others will not," he said in a statement. "We have no plans at this time to change the national threat level, although we remind everyone that the aviation threat level has been raised to orange since last fall," Chertoff said. Orange is the second highest of five levels on the US terror alert chart and indicates a high risk of terrorist attacks. The current national threat level is yellow, or the third highest, indicating an elevated threat. The Transportation Security Administration is posting more agents outside terminals at some airports, Snow said. "There will be some inconvenience of passengers in terms of longer wait times," Snow said. Local police also may take separate measures, he added. Police stepped up curbside patrols with canine units at Hartsfield-Jackson Atlanta International Airport in Georgia. The Port Authority of New York and New Jersey, which operates Newark Liberty in New Jersey and John F. Kennedy and LaGuardia airports in New York, took "a number of measures as we always do to respond to security situations immediately," spokesman Steve Coleman said. At Los Angeles International Airport, additional police, including canine units, were on duty Saturday, said spokesman Marshall Lowe. Operations at Miami International Airport went on heightened alert through at least July 4, and in Philadelphia officials were increasing patrols around the perimeter of the airport. At Washington's Reagan National and Dulles International airports, spokesman Ron Yingling said some measures are behind the scenes. "I don't think there's anything different in what passengers have to physically do to get through security that's different from yesterday." Bush, who spent the day biking and fishing, was kept abreast of the developments in Britain, Snow said. US officials were in contact with their counterparts in Europe, Snow said. FBI spokesman Richard Kolko said the bureau stood ready to help British authorities. Two men rammed a flaming sport utility vehicle into the main terminal of the airport in Glasgow, Scotland, crashing into the glass doors at the entrance and causing a fire, witnesses said. Police said four suspects were arrested. On Friday, British police thwarted a plot to bomb central London, discovering two cars abandoned with loads of gasoline, gas canisters and nails. Bush was in Maine to host Russian President Vladimir Putin at the Bush family's summer home on the coast. Putin was to arrive Sunday for a two-day stay.