US jihadist fighters returning from conflict zones pose a “very serious threat” to US national security alongside British and Canadian nationals that also fought oversees as they can freely enter the American soil, top politicians say.

It is impossible to track every single person who might have visited a conflict zone such as Syria or Iraq, chairman of the House Intelligence Committee, Mike Rogers said, expressing concerns over American, British and Canadian jihadist fighters who potentially can pose a very serious threat to the US.

“I'm very concerned because we don't know every single person who has gone and trained and learned how to fight,” Rogers told Fox News Sunday, urging the White House to aggressively prosecute Americans who had trained overseas.

Hundreds of US citizens had gone overseas, Rogers said, in addition to some 500 British citizens and hundreds more from Canada.

“The chances of error are greater than our ability to track every single area. It's a very serious threat,” he said.

Meanwhile, he noted, the US is tracking “pretty serious” threats of planned attacks in the West by al-Qaeda.

Another member of the House Intelligence Committee, Representative Dutch Ruppersberger, echoed Rogers' assessment.

“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, referring to a man who became first known US suicide bomber after blew himself up in an attack in Syria in May.

On Saturday, US Secretary of State John Kerry called for an international coalition to combat the Islamic State and its “genocidal agenda” on a larger scale, as the US continues to hit jihadist positions in Iraq in limited airstrikes.

READ MORE:Facing ‘abstract threat’, Germany to arm Iraqi Kurds against Islamic State

Over in Europe, Germany, which estimates to have at least 400 of its nationals fighting alongside extremist forces announced that it is facing an “increased abstract threat” while the government approved $70 million budget for arming Kurdish forces deterring IS in Iraq.

READ MORE:UK raises terror threat to 'severe', Cameron says 'attack likely'

In Britain, meanwhile on Friday, authorities raised the terror alert level from “substantial” to “severe” over fears of possible jihadist attacks. A response is needed urgently, said British PM David Cameron, as the UK and its allies “could be facing a terrorist state on the shores of the Mediterranean bordering a NATO member.”