Foreign fighters are streaming into Syria and Iraq in unprecedented numbers to join the Islamic State or other extremist groups, including at least 3,400 from Western nations among 20,000 from around the world, U.S. intelligence officials say in an updated estimate of a top terrorism concern.

Intelligence agencies now believe that as many as 150 Americans have tried and some have succeeded in reaching in the Syrian war zone, officials told the House Homeland Security Committee in testimony prepared for delivery on Wednesday. Some of those Americans were arrested en route, some died in the area and a small number are still fighting with extremists.

The testimony and other data were obtained Tuesday by The Associated Press. […]