The officials, speaking on condition of anonymity, said that proposals circulating within government agencies could make it easier for the government to revoke U.S. passports.

Authorities in the United States, Canada, Australia and western Europe say they are alarmed by thousands of citizens who have traveled to Syria to fight in the conflict there. They fear that some battle-hardened fighters could return to their home countries and attempt terror attacks.

Secretary of State John Kerry, whose department issues passports, has authority to revoke them. The government regards passports as a privilege, not a right. But one official said there are provisions for challenging such decisions.