The U.S.-led coalition's airstrikes against the Islamic State group in Syria and Iraq have failed to halt the momentum of foreign fighters traveling to the Middle East to join extremist organizations like ISIS, a report said Tuesday. About 20,000 fighters from around the world -- 3,400 from Western countries -- are estimated to have joined these groups.

As many as 150 Americans are believed to have tried to reach Syria in recent times, The Associated Press (AP) reported, citing officials who provided the information to the House Homeland Security Committee in a testimony expected to be delivered Wednesday. While some of those American citizens succeeded, others were arrested along the way, it said.

According to the prepared testimony, the estimate of 20,000 foreign fighters from 90 countries is up from an earlier estimate of 19,000 fighters. The number of Americans who have gone or tried to go to Syria and Iraq is up from 50 a year ago and 100 by the fall of 2014.

The Syrian war has created “the largest convergence of Islamist terrorists in world history,” Rep. Michael McCaul, R-Texas, the chairman of the committee, reportedly said in the testimony, adding that coalition airstrikes have been unable to stem the influx of foreign fighters into the region.

The committee has compiled a list of 18 U.S. citizens who have joined or attempted to join ISIS, while 18 others tried or succeeded to join other Islamic groups. The list also includes three teenagers each from Chicago and Denver, who are believed to have been radicalized and recruited online. All of them were arrested on their way to Middle East conflict zones, AP reported.

ISIS has reportedly formed the “Anwar al-Awlaki Battalion,” a unit composed only of English-speaking foreign fighters whose purpose is to plan and execute attacks in English-speaking countries. Awlaki, an American-born al Qaeda leader and recruiter, was killed by a U.S. drone strike in Yemen in 2011, and is revered among foreign fighters in ISIS' ranks.

Meanwhile, U.S. President Barack Obama is expected to seek authorization from Congress on Wednesday to launch a new three-year plan to use force against ISIS, though with limits on U.S. combat troops' involvement, Reuters reported, adding that the proposal will allow the use of special forces and advisers for defensive purposes, but prohibit “enduring offensive ground forces.”