The US State Department says thousands of foreign militants, including some Americans, have traveled to Syria to fight the government there.

The US State Department says thousands of foreign militants, including some Americans, have traveled to Syria to fight the government there.

"We think that there are approximately 12,000 fighters from at least 50 countries in Syria - foreign fighters, including a small number of Americans - that may have travelled to Syria since the beginning of the conflict" more than three years ago, said deputy State Department spokeswoman Marie Harf on Thursday.

"They may all not still be there," she added without revealing an estimated number of Americans.

Meanwhile, another US official, who was speaking on condition of anonymity, said more than 100 US citizens have traveled or tried to travel to Syria to fight the government there.

A day earlier, FBI Director James Comey said uprooting homegrown terrorists is the FBI's number one priority. He added that Americans who have tried to join the insurgency in Syria pose a serious security threat to the US homeland.

They have travelled to join terrorist groups including ISIL which is controlling vast swaths of land straddling the border between Syria and Iraq.

The brutal beheading of American journalist James Foley by a Briton fighting in the ranks of ISIL is the latest and most shocking example of foreign militants threatening western interests.

A grisly video posted online Tuesday and later confirmed as "authentic" by the White House shows an ISIL militant decapitating Foley in Syria in retaliation for US airstrikes against the terrorist group in northern Iraq. The group has threatened to kill another American hostage if its demands are not met.