U.S. extra vigilant after British terror scares, Chertoff says

(CNN) -- Foiled terror attacks in Britain and a history of summertime terrorism have U.S. authorities practicing extra vigilance, Department of Homeland Security Secretary Michael Chertoff said Monday.

Chertoff said it's no secret that al Qaeda wants to launch another big attack against the United States, but he has seen nothing about a specific plot.

"I'm not going to talk about specific documents or intelligence, but I can say to you things I, frankly, have said publicly for some period of time. We know al Qaeda is still very interested in carrying out attacks against the United States and Western Europe," Chertoff told CNN's "American Morning."

"We know that there have been an increase in public statements by al Qaeda leaders recently which suggested they are raising some of their profile, and we know traditionally the attacks that they plan are very significant and intended to cause a huge amount of damage, as witness the [London] airline plot of last August which, had it been successful, would have resulted in the detonation of bombs on multiple airliners.

"We've seen, over the past two summers and now this summer, an attack in England. All of those things put together do cause us to be concerned going into the summer." Watch how enhanced security means long lines for holiday travelers »

Chertoff told Fox News that terrorism directed through Europe remains a particular concern.

"We have an increasing concern about people who might be training in Pakistan or Afghanistan or in South Asia and who might then be traveling to Europe, where they have European documents because they're European citizens, and then might use Europe as a platform to launch an attack against the U.S.

"And we've taken some very specific measures to increase the amount of information we're gathering on passengers precisely for that reason," he said.

He told CNN that security has been increased at airports and other transportation hubs, but he noted that the threat level for aviation has been at orange since last August, "the highest level, other than what happens when you have an actual attack."

Meanwhile, the New York Police Department has beefed up its presence, though there have been no threats made against the city, Commissioner Ray Kelly said Monday.

Without a specific threat to which to respond, there is little authorities can do except increase their numbers -- especially in places such as subway platforms, parking garages, bridges, tunnels and high-traffic areas such as Times Square and the theater district -- "but the functions will remain the same," Kelly said.

Chertoff's Monday remarks were echoed by two U.S. counterterrorism officials who said that since 9/11 there has been fear that al Qaeda will hit the United States hard again, possibly with weapons of mass destruction. But these officials said there is nothing suggesting that this summer is more likely than any other time.

An additional U.S. counterterrorism official said, "We continue to pick up chatter -- but would not identify it as a spike. There is nothing that we are seeing that parallels 9/11." E-mail to a friend

CNN's Kelli Arena, Pam Benson, Sarah B. Boxer and Kiran Chetry contributed to this report.