Updated

The United States will not restrict visa-free travel for European tourists, despite a growing number of terrorist plots involving European citizens, the director of the FBI said Monday.

Robert S. Mueller said the United States had to strike "a difficult balance" between openness and security, but insisted there were no plans to cut back the visa waiver program that allows citizens of many European Union countries to enter the United States without visas.

"If you had had the same discussions as I have with my State Department you would know that nothing is going to happen to visa waiver in the near future," Mueller told an audience at the Chatham House think tank in London. "In fact there are additional countries that are being admitted to that program."

The visa waiver program currently excludes Greece and most of the eastern European nations that joined the EU in 2004, although countries including Estonia, Latvia and the Czech Republic have recently signed bilateral deals with Washington that should qualify them for the program later this year.

EU officials hope to persuade the United States to extend the program to all 27 members of the bloc, but have resisted U.S. requests that they in turn provide more passenger data and adopt new security measures, including air marshals on trans-Atlantic flights.

Mueller acknowledged that a number of foiled terrorist plots involved European nationals. Eight British men are currently on trial in London for allegedly plotting to blow up trans-Atlantic passenger planes in mid-air.

Mueller said he could not comment on that case while the trial was under way, but conceded that the EU's relaxed borders and other factors presented "a challenge" for U.S. security officials.

"We are where we are with visa waiver. We are where we are with the European Union," Mueller said. "The breakdown of the borders occurred 10 or 15 years ago and we will never retrench on that, and probably we should not."

Mueller told an audience of academics, security experts and journalists that he believed the battle against al-Qaida inspired terrorism would be won in a matter of years, rather than decades.

"I don't think it will be millennia or generations," said Mueller, who was appointed FBI director days before the Sept. 11, 2001 attacks on New York and Washington. "I think we will see victory on my watch."