WASHINGTON: Two key US lawmakers are urging the Obama administration to take a tougher stance against Islamic State (IS) militants, with one legislator warning that hundreds of US, British and Canadian citizens who have trained with the extremist group pose a “very serious threat” to the United States.

Representative Mike Rogers, chairman of the House Intelligence Committee, said the administration needs to aggressively prosecute US citizens who had trained overseas as it weighed options for how to respond to escalating violence by IS militants in Iraq.

Democratic Sen. Dianne Feinstein said President Barack Obama may be “too cautious” in his approach in dealing with the extremist group, which is trying to carve out a “caliphate” in Iraq and Syria.

The chair of the Senate Intelligence Committee, Feinstein teold NBC’s “Meet the Press” that the Defense and State departments have been putting together a response to the threat.

She said she has seen nothing to compare to the viciousness of the militants who have overrun large portions of Iraq and Syria, killed civilians and beheaded American journalist James Foley.

Feinstein says the Islamic State group has financing, military structure and weapons unlike any other militants and calls them “extraordinarily dangerous.”

The United States has carried out three air strikes on Saturday against IS fighters near the besieged Shiite town of Amerli in northern Iraq and airdropped more than hundred additional bundles of humanitarian aid to civilians trapped there.

President Barack Obama last week authorized the new military action, broadening US operations in Iraq amid an international outcry over the threat to Amerli’s mostly ethnic Turkmen population. Washington is seeking to build an international coalition to fight against Islamic State forces.

But Rogers said he believed hundreds of US citizens had trained with Islamic State at least once, in addition to an estimated 500 British citizens and hundreds more from Canada, with their passports essentially allowing them “free travel” to the United States to carry out extremist attacks.

“The chances of error are greater than our ability to track every single area. It’s a very serious threat,” he said, noting that US intelligence agencies were also tracking “pretty serious” threats of planned attacks in the West by Al-Qaeda.

Concerns about Americans associating with IS militant fighters have grown in recent weeks after the deaths of two US citizens in Syria.

US Attorney General Eric Holder said in July dozens of Americans were among roughly 7,000 foreign fighters that US intelligence agencies estimate to be operating in Syria, out of roughly 23,000 violent extremists.

Rogers said the number of Americans who had traveled and trained overseas at least once was considerably higher.

He said US efforts to tamp down on such travel by US citizens needed to be handled carefully to avoid restricting the ability of other Americans to travel overseas.

But aggressive prosecution of those who trained with extremist groups could have a deterrent effect, he said.

Representative Dutch Ruppersberger, the top Democrat on the House Intelligence Committee, echoed Rogers’ concerns.

“The biggest threat that I see to the United States right now are Americans and Brits who have passports that have the ability to come into our country without getting a visa,” Ruppersberger told CNN’s “State of the Union” program.

“We had the suicide American bomber who was radicalized, came home to visit his parents, went back and then killed himself. Now, that could have happened in the United States,” Ruppersberger said.

A 22-year-old man from Florida carried out a suicide bombing in Syria’s Idlib province in May. A Denver woman was arrested in July on suspicion of trying to fly to Syria to help insurgents, and two men in Texas were arrested on similar charges in June.