Foreign fighters are flocking to Syria at an "unprecedented" rate, with more than 20,000 volunteers from around the world joining the Islamic State or other extremist groups, US intelligence officials said Tuesday.

The foreign fighters have traveled to Syria from more than 90 countries, including at least 3,400 from Western states and more than 150 Americans, according to the latest estimate from the National Counter-Terrorism Center (NCTC).

A majority of the foreign volunteers who arrived recently have joined forces with the Islamic State group in Syria and Iraq, it said.

The estimate of the total number of foreign fighters flocking to Syria was up from a previous estimate in January of 19,000, according to NCTC.

No precise numbers are available "but the trend lines are clear and concerning," Nicholas Rasmussen, NCTC director, said in prepared remarks for a hearing before lawmakers on Wednesday.

"The rate of foreign fighter travel to Syria is unprecedented. It exceeds the rate of travelers who went to Afghanistan and Pakistan, Iraq, Yemen, or Somalia at any point in the last 20 years," he said.

The profile of those who head to the Syrian conflict are from a range of backgrounds and "do not fit any one stereotype," Rasmussen said.

"The battlefields in Iraq and Syria provide foreign fighters with combat experience, weapons and explosives training, and access to terrorist networks that may be planning attacks which target the West," he said.

Western governments have voiced increasingly alarm over the flow of foreign volunteers heading to the Syrian conflict, particularly in the aftermath of jihadist attacks in Paris that left 17 dead.

The director's prepared testimony for the House Homeland Security Committee was released to AFP on Wednesday.