Of those 3,400 ISIS (IS, ISIL) fighters coming from the Western world, as many as 150 Americans may have reached the battlefield, officials told the House Homeland Security Committee in testimonies prepared for delivery on Wednesday, according to AP. Others were arrested en route to the war zones, while some died in the area.

The chief of the National Counterterrorism Center, Nick Rasmussen, said the number of foreign fighters joining the terrorists in Syria and Iraq is growing at an unprecedented rate in comparison to foreign militants who went to Afghanistan, Pakistan, Iraq, Yemen, or Somalia in the past 20 years. The overall estimate of 20,000 foreigners in ISIS's army has been revised from 19,000, according to Rasmussen’s prepared testimony.

The chairman of the committee, Texas Republican Rep. Michael McCaul, claimed that according to agencies, the “Islamic caliphate” is “the largest convergence of terrorists in world history.” Sustained bombing by the American coalition has not stopped the inflow, he noted.

In another testimony prepared for Wednesday, the Homeland Security Department’s intelligence chief, Francis X. Taylor, said Washington is “unaware of any specific, credible imminent threat to the Homeland.”

But Michael Steinbach, assistant director of the FBI, will brief the lawmakers that last month the ISIS released a video encouraging “lone wolf” radicals – those who have not yet traveled to Syria or Iraq – to instead attack soldiers, police, and intelligence officers in Western countries, Reuters reported

“Once in Syria, it is very difficult to discern what happens there,” according to Steinbach's testimony. “This lack of clarity remains troubling.”

Yet the flow of fighters – who hail from some 90 countries – is irregular, another official who spoke on condition of anonymity told Reuters.

Meanwhile, the White House wants Congress to authorize the US military to fight the ISIS over the next three years, and is reportedly planning to announce a formal request on Wednesday.