Hundreds of Westerners, including a dozen or more Americans, are fighting with the Islamic State (IS) and posing a security challenge to the Western countries, the U.S. State Department said on Monday.
"We are particularly concerned about the Westerners and even the small number of Americans who have crossed the border, gone to fight with IS and have a Western passport," State Department spokeswoman Marie Harf said at a press briefing in Washington.
About 15,000 foreign fighters from more than 80 countries may have joined the IS, and at least 2,000 Westerners are fighting with different groups in Iraq and Syria, said Harf, adding that the U.S. is "very focused from a security perspective on that."
"When it comes to the IS threat to the homeland, the most concerning thing is Westerners with these passports, particularly Americans, but also Europeans," Harf said. "That really is the biggest challenge right now we' re facing."
Harf said the U.S. could revoke passports of American IS fighters and put additional screenings on European fighters to prevent them from coming to the United States or Europe.
According to CIA' latest estimate, the IS has between 20,000 and 31500 fighters in Iraq and Syria.
At the United Nations General Assembly in late September, U.S. President Barack Obama will chair a session on combatting foreign fighters who have joined extremist groups in Iraq and Syria.