Dozens of Americans have joined terrorist groups and are posing a threat to the United States and its interests abroad, the president’s most senior adviser on counterterrorism and homeland security said Thursday.

“There are, in my mind, dozens of U.S. persons who are in different parts of the world, and they are very concerning to us,” said John O. Brennan, deputy White House national security adviser for homeland security and counterterrorism.

In a wide-ranging interview with The Washington Times, Mr. Brennan said he would not talk about lists of targeted American terrorists. However, U.S. intelligence and law enforcement agencies have been tracking down U.S. nationals and U.S. passport holders who pose security threats, like the Yemen-based al Qaeda cleric Anwar al-Awlaki, he said.

“They are concerning to us, not just because of the passport they hold, but because they understand our operational environment here, they bring with them certain skills, whether it be language skills or familiarity with potential targets, and they are very worrisome, and we are determined to take away their ability to assist with terrorist attacks,” Mr. Brennan said.

The remarks came in response to questions about procedures used by the president to order lethal strikes on U.S. citizens who have joined al Qaeda or other terrorist groups.

On Feb. 3, Dennis C. Blair, then director of national intelligence, said in congressional testimony that special permission must first be obtained by military or intelligence forces before what he termed “direct action” strikes against American citizens.

The main weapon in recent CIA and U.S. military counterterrorism operations has been attacks with missile-equipped unmanned aerial vehicles in Afghanistan, Iraq, Pakistan, Somalia and Yemen. The administration has said it has killed dozens or perhaps scores of terrorists with these strikes over the past several years.

That practice was criticized in a report earlier this month authored by Philip Alston, the independent U.N. investigator on extrajudicial killings, who said the practice may violate international humanitarian law.

The American Civil Liberties Union in a letter to Mr. Obama on April 28 warned that the current program to kill terrorists in foreign countries would create a precedent for other countries to kill suspected terrorists all over the world.

The American-born cleric and U.S. citizen who now resides in Yemen is thought to be high on the list of those targeted for killing by the United States.

Mr. Brennan would not comment on the details of lethal operations or the procedure for targeting Americans.

“If a person is a U.S. citizen, and he is on the battlefield in Afghanistan or Iraq trying to attack our troops, he will face the full brunt of the U.S. military response,” Mr. Brennan said. “If an American person or citizen is in a Yemen or in a Pakistan or in Somalia or another place, and they are trying to carry out attacks against U.S. interests, they also will face the full brunt of a U.S. response. And it can take many forms.”

Mr. Brennan added, “To me, terrorists should not be able to hide behind their passports and their citizenship, and that includes U.S. citizens, whether they are overseas or whether they are here in the United States. What we need to do is to apply the appropriate tool and the appropriate response.”

Attempts by U.S. citizens at carrying out unsophisticated terrorist attacks in the United States have increased sharply in recent years. The latest example was Faisal Shahzad, who confessed in court this week that he left a vehicle rigged with explosives in New York’s Times Square on the evening of May 1. In court testimony, he also admitted to having trained in bomb making with the Pakistani Taliban.

A recent Rand Corp. study of so-called “homegrown” U.S. radicalism reported a significant increase in indictments of Americans who were recruited for jihadist violence in the past two years.

The report said 81 were indicted for terrorism-related crimes between 2002 and 2008. Forty-two people were indicted for such crimes in 2009, and two more have been indicted in 2010.

The study, authored by former U.S. special-operations officer Brian Jenkins, concluded that one in 30,000 Muslim Americans is vulnerable to radicalization, a fact “suggesting an American Muslim population that remains hostile to jihadist ideology and its exhortations to violence.”

In the interview Thursday, Mr. Brennan also said that the vision of Islam put forth by Osama bin Laden and other al Qaeda leaders was widely rejected by the Muslim world. Last month, Mr. Brennan drew criticism for a speech in which he said, “Jihad is holy struggle, a legitimate tenet of Islam, meaning to purify oneself or one’s community.”

Mr. Brennan said that he opposed granting any legitimacy to what he called al Qaeda’s “twisted” interpretation of Islam.

“Clearly, bin laden and al Qaeda believe they are on this very holy agenda and this jihad,” he said. “However, in my view, what we cannot do is to allow them to think, and the rest of the world to think, and for the future terrorists of the world to believe al Qaeda is a legitimate representation of jihad and Islam.”

Mr. Brennan also said that the U.S. law enforcement community has the means to monitor Web forums affiliated with al Qaeda that have in the past proven to be a gateway for recruitment into the terrorist organization.

But he also said that any investigations or monitoring of such sites needed to first pass a threshold of probable cause.

“There needs to be some type of predicate or premise for there to be reasonable suspicion that someone is engaged in activity that is unlawful,” he said. “The mere engagement in political speech, even if it is radical, is not in itself a cause for investigation.”

Mr. Brennan toward the end of the interview acknowledged that, despite some differences, there is considerable continuity between the counterterrorism policies of President Bush and President Obama.

“There has been a lot of continuity of effort here from the previous administration to this one,” he said. “There are some important distinctions, but sometimes there is too much made of those distinctions. We are building upon some of the good foundational work that has been done.”

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