Security on America's mass transit systems has been stepped up following a threat to New York's subways but travelers everywhere are warned today to remain vigilant against terrorist attack especially as the holidays draw near.

Intelligence experts believe that it is only a matter of time before there is another terrorist attack in the U.S. and mass public transport systems and holiday resorts are both prime targets. Both are easy to attack and difficult to defend and both could lead to mass casualties and major disruption, said Don Philpott, of IsAmericaSafe.com, a website dedicated to protecting Americans and the Homeland.

"The recent deadly terrorist bombings in Bali were designed to disrupt tourism and hit the economic wellbeing of that country. Similar attacks were recently carried out at a popular tourist resort in Egypt for the same reasons. Other attacks took place in London and Madrid with the aim of causing death and destruction and striking terror among the millions of people who daily use the mass transport systems in the two capitals", he said.

Americans travel on public transport more than 32 million times a day and subways are particularly vulnerable targets to bombs or biochemical attack with hundreds of passengers crowded into a very confined space often well below ground, said Philpott.

"How long will it be before an attack is carried out in the US either at a top tourist destination or on a mass transport system? That is why we all have to be vigilant and play our role in defending the Homeland," he warned.

"This means reporting anything that you think is suspicious and being alert to what is going on around us at all times. If you are traveling on any form of mass transport system keep an eye on what others are doing - just in case - and report any suspicious parcels or packages. By being vigilant we will help protect ourselves and others," he said,

New York Metropolitan Transportation Authority's Director of Security William A. Morange, echoed the call, and said that the help of passengers is critically important to maintaining a high level of security for the regional public transportation system.

"It is impossible for the police departments to be everywhere and see everything. Our passengers extend our reach and-by sharing their information-make the system safer, he said."