International intellegence teams are continuing to roundup suspects believed to be behind a series of terror plots aimed at Europe.

While the threat targets cities overseas, officials in the U.S. are urging Americans to pay closer attention and be on the look out for suspicious activity.

Tuesday, the heads of America's largest mass transit systems were briefed on the overseas threat. Security has increased on Amtrak trains and in New York City.

"We've done some tactical shifts and changes that we think are appropriate at this time," New York City Police Commissioner Ray Kelly said.

CBN News Terrorism Analyst Erick Stakelbeck has more on the threat in Europe and how it affects the U.S.  Click play for his comments, following Dale Hurd's report.

The travel alert for Americans in Europe is light on details, but heavy on impact:

"I wouldn't say that we have specific information about a particular place, a particular time," U.S. Attorney General Eric Holder said. "And yet we have sufficient information that justifies the issuing of the alert."

Monday, the U.S. attacked a suspected center of the European terror plot in Pakistan, killing at least five people at a training camp where German citizens had been recruited to carry out terrorist acts.

"Several people who were involved in the plotting have been targeted and the others are certainly on the radar of U.S., Pakistani and European intelligence services," Pakistani Ambassador Husain Haqqani explained.

Meanwhile, 11 people were apprehended Tuesday in southern France in sweeps against suspected Islamic militant networks.

"If al Qaeda is planning simultaneous attacks in Europe, there's nothing to say they could not also include the U.S. on that list of simultaneous attacks," said Richard Clarke, former White House national security fofficial and now an ABC News consultant.

In response to the threat, the Department of Homeland Security recently produced videos for the retail industry, sports arenas, and hotel employees on safety and spotting suspicious behavior.

The move is part of a greater campaign by the U.S. government to make the American public aware that the terror threat is growing.

Friday, Amtrak will hold a high security exercise called "RailSafe," in which there will be an increased security presence along national transit routes.