The United States has warned that Americans joining extremist fighters in Syria are posing a real threat at home.

The United States has warned that Americans joining extremist fighters in Syria are posing a real threat at home.

Federal officials said some US militants are taking part in the Syrian conflict, raising the chances they could become radicalized by al-Qaeda-linked militant groups and return to the US as battle-hardened security risks.

The State Department said it has no estimates of how many Americans are fighting against the Syrian government. Other estimates, from an arm of the British defense consultant IHS Jane’s and from experts at a nonprofit think tank in London, put the number of Americans at a couple dozen.

This year, at least three Americans have been charged with planning to fight beside the terrorist group, Nusra Front. The most recent case involves a Pakistan-born North Carolina man arrested on his way to Lebanon.

At a Senate homeland security committee hearing this month, Sen. Thomas Carper, D-Del., said: “We know that American citizens as well as Canadian and European nationals have taken up arms in Syria, in Yemen and in Somalia. The threat that these individuals could return home to carry out attacks is real and troubling.”

The hearing came about two weeks after the FBI and other officers arrested Basit Sheikh, 29, at the Raleigh-Durham International Airport on charges he was on his way to join Nusra Front.

Sheikh, a legal resident of the United States, had lived quietly, without a criminal record, in a Raleigh suburb for five years before his Nov. 2 arrest. A similar arrest came in April in Chicago. And in September, authorities in Virginia released an Army veteran accused of fighting alongside the group after a secret plea deal.

In August, outgoing FBI Director Robert Mueller told ABC News that he was concerned about Americans fighting in Syria, specifically “the associations they will make and, secondly, the expertise they will develop, and whether or not they will utilize those associations, utilize that expertise, to undertake an attack on the homeland.”

Current FBI Director James Comey said this month that he worried about Syria becoming a repeat of Afghanistan in the 1980s, after the Soviet invasion, with foreign fighters attracted there to train.