WASHINGTON (Reuters) - The FBI is concerned that Americans may be fighting in Syria and could bring terrorist tactics back to the United States, but U.S. officials say there is a small number of U.S. citizens fighting with Syrian rebels.
FBI Director Robert Mueller told ABC News in an interview aired on Friday that the terrorism threat that began in Afghanistan and Pakistan has now "migrated" to places including Syria, Libya, Egypt and Yemen.
"You have individuals traveling to those venues, you are concerned about the associations they will make, and secondly about the expertise they will develop and whether or not they will utilize those associations, utilize that expertise, to undertake an attack upon the homeland," Mueller said in the interview.
"So, yes, we are concerned about that, and, yes, we are monitoring it," he said.
Places like Syria may end up harboring "radical extremists who want to do harm" to the United States, according to Mueller, who will leave his FBI post next month after 12 years.
U.S. government officials and private experts monitoring the 2-1/2-year-old civil war in Syria have said Americans and other foreigners were taking part in the fighting to oust President Bashar al-Assad. Some experts said it had become a pilgrimage destination for Sunni Muslim militants.
One U.S. national security official estimated on Friday that only a handful of Americans are currently fighting in Syria, although other officials have said several dozen U.S. citizens have cycled through the country since fighting began.
An American woman was killed this year in the company of Syrian rebels in Idlib province.
A former U.S. soldier was arrested on his return to the United States in March and charged with conspiring to use a rocket-propelled grenade in Syria. Investigators said he acknowledged fighting with Syrian rebels, including the Nusra Front, which Washington says is a branch of al Qaeda.
U.S. and European security officials have said they are more concerned about the traffic in and out of Syria of would-be fighters from European countries, including Britain, France and Germany. European nationals often can easily enter the United States without obtaining an advance visa.
European officials have said that at any one time - including the present - they believe that as many as 70-100 British citizens are fighting in Syria with anti-Assad rebels, many of them with the Nusra Front.
Due to permissive European travel rules and Syria's geographical location, counter-terrorism officials say it is difficult to track the movements of European citizens, and sometimes Americans, who go to Syria to fight with rebels.