The United States is increasing transportation security after a car bombing and other attempted terror attacks in Britain in recent days. The top U.S. security official says although he sees no links to any specific attacks in the United States, he is worried about what he calls terrorist copy cats. VOA's Stephanie Ho reports from Washington.

Traffic backs up at a vehicle checkpoint at Los Angeles International Airport after security measures were tightened following bombing attempts in Britain

He told ABC's This Week program there is no indication of a linkage between the events in Britain and any specific attacks on the United States. "We do not have, at this point, specific, credible intelligence that there is an attack, a particular attack, focused on this country," he said.

But at the same time, speaking on CNN's Late Edition program, Chertoff said U.S. authorities are concerned about the threat of terrorism, especially in the coming months. "We have had a number of al-Qaida leaders quite publicly talking about how they want to carry out their threats against the West. So, given that and given the history that we have had over past summers, with attacks in Britain, I think we were clearly focused on this as a possibility," he said.

Chertoff said the United States will enact additional security measures at airports, train stations and other mass transit stations this week, as the country prepares to celebrate the July Fourth Independence Day holiday.

He added that there will be what he described as "enhanced air marshal work" on trans-Atlantic flights, with special attention to Britain. U.S. air marshals have been accompanying U.S.-bound flights since a plot to blow up airliners between the United States and Britain was uncovered last August in London.

Chertoff said Washington is increasingly concerned about people with European citizenship going to South Asia to get terrorism training, and then returning to carry out attacks in Europe or the United States. He told CNN there are, what he described as, a "significant number of people" within the United States who have suspected links to terrorism. "I am not saying those are linked to al Qaida, but they certainly indicate the possibility of people becoming radicalized and deciding they want to carry out attacks on their own. So, this is not, by no means, a European problem, but a problem here as well," he said.

Chertoff said he has no immediate plans to raise the overall threat level in the United States, which is at yellow, signifying an elevated level. Since last August, the U.S. threat level for all domestic and international flights has been more serious, at level orange.