Last Updated Feb 10, 2015 9:55 PM EST
WASHINGTON -- Foreign fighters are streaming into Syria and Iraq in unprecedented numbers to join the Islamic State or Iraq and Syria (ISIS) 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. A U.S. intelligence official confirmed the information to CBS News.
Nick Rasmussen, chief of the National Counterterrorism Center, said the rate of foreign fighter travel to Syria is without precedent, far exceeding the rate of foreigners who went to wage jihad in Afghanistan, Pakistan, Iraq, Yemen or Somalia at any other point in the past 20 years.
U.S. officials fear that some of the foreign fighters, who come from 90 countries, will return undetected to their homes in Europe or the U.S. to mount terrorist attacks. At least one of the men responsible for the attack on a satirical magazine in Paris had spent time with Islamic extremists in Yemen.
Meanwhile, the White House circulated a proposal Tuesday that would have Congress authorize the U.S. military to fight ISIS terrorists over the next three years. A formal request for legislation is expected on Wednesday.
Also at the White House, President Barack Obama praised Kayla Jean Mueller, the young American whose death was confirmed Tuesday. Mueller died while in ISIS hands, though the group blamed a Jordanian airstrike, and Obama said, "No matter how long it takes, the United States will find and bring to justice the terrorists who are responsible for Kayla's captivity and death."
As for foreign fighters, officials acknowledge it has been hard to track the Americans and Europeans who have made it to Syria, where ISIS is the dominant force trying to overthrow the government of President Bashar Assad. The U.S. Embassy in Syria is closed, and the CIA has no permanent presence on the ground.
"Once in Syria, it is very difficult to discern what happens there," according to Wednesday's prepared testimony of Michael Steinbach, the FBI's assistant director for counterterrorism. "This lack of clarity remains troubling."
The 20,000-fighters estimate is up from 19,000, Rasmussen will tell the House committee, according to prepared testimony. The number of Americans or U.S. residents who have gone or tried to go is up to 150 from 50 a year ago and 100 in the fall.
Rep. Michael McCaul, the Texas Republican who chairs the committee, said in his prepared remarks that the Syrian war had created "the largest convergence of Islamist terrorists in world history." Sustained bombing by a U.S.-led coalition has not stopped the inflow, he noted.
"I am worried about our ability to combat this threat abroad, but also here at home," McCaul said, according to a copy of his prepared remarks obtained by CBS News. "I wrote to the president recently as part of my ongoing investigation and raised concerns that we have no lead agency in charge of countering domestic radicalization and no line item for it in the budgets of key departments and agencies. I am also concerned that the few programs we do have in place are far too small to confront a challenge that has grown so quickly."
McCaul's committee staff compiled from public sources a list of 18 U.S. citizens or residents who joined or attempted to join the Islamic State group, and 18 others who tried to or succeeded in joining other violent Islamic groups.
U.S. intelligence officials do not make public their estimate of how many Americans currently are fighting in Syria and Iraq. In September, FBI director James Comey said it was "about a dozen."