Nearly 30,000 foreign recruits have now poured into Syria, many to join the Islamic State, a doubling of volunteers in just the past 12 months and stark evidence that an international effort to tighten borders, share intelligence and enforce antiterrorism laws is not diminishing the ranks of new militant fighters.

Among those who have entered or tried to enter the conflict in Iraq or Syria are more than 250 Americans, up from about 100 a year ago, according to intelligence and law enforcement officials…

A year ago, the same officials estimated that flow to be about 15,000 combatants from 80 countries, mostly to join the Islamic State.