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 roughly 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 congressional hearing 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 volunteers come 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 increasing 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.

In the months-long battle for the Syrian town of Kobane near the Turkish border, large numbers of foreign fighters were among the jihadists killed, according to US officials.

Kurdish forces, backed up by US-led air strikes, eventually succeeded in fending off an attempt by the IS group to seize Kobane.

- Propaganda appeals -

The IS militants are able to recruit new volunteers partly because of their savvy use of propaganda on social media, producing videos and appeals in a range of languages, Rasmussen said.

Apart from grisly images of murders of hostages and battlefield executions, the group also tries to reach alienated youth by promoting images of a welcoming, "bucolic" life in their self-declared caliphate, he said.

Catering to a younger, thrill-seeking audience, the IS jihadists employ references to Western brands and popular video games, he said.

"They have also coined pithy 'memes' such as, 'YODO: You Only Die Once. Why not make it martyrdom?'"

Al-Qaeda and its branches in the Middle East and Africa have never displayed such an acumen with propaganda, he added.

The NCTC director's prepared testimony for the House Homeland Security Committee, which holds a hearing on Wednesday, was released to AFP on Tuesday.

There was no single route the foreign fighters travel to reach Syria, but most eventually pass through Turkey "because of its geographic proximity to the Syrian border areas," he said.

The recruits have taken advantage of Turkey's visa-free travel arrangements with about 69 governments, including with European Union states, the director said.

Turkey has bolstered its effort to stem the flow and deny entry to potential foreign fighters. The country now has a travel ban list that includes some 10,000 people.

But while Turkey and other European countries have strengthened border controls and taken other steps, "significant work remains" to prevent volunteers from heading to Syria or to stop them from returning, he said.

In the end, the only way to counter extremist threats and the IS group is to "diminish the appeal of terrorism and dissuade individuals from joining them in the first place," Rasmussen said.

In a statement for Wednesday's hearing, the Republican chairman of the House committee, Michael McCaul, said he was "worried about our ability to combat this threat abroad, but also here at home."

The threat of homegrown extremism, in which individuals inspired by Islamist propaganda are motivated to launch attacks, also remains cause for concern but has not intensified, he said.

The NCTC believes the annual threat of homegrown violence could result in fewer than 10 "uncoordinated and unsophisticated" plots in the United States from a pool of up to a few hundred people, he said.