U.S. Department of Homeland Security Secretary Jeh Johnson has admitted that terrorist organisations like the Islamic State (ISIS) may use the refugee programme of the Obama administration to enter the United States.

Johnson told reporters in Washington, D.C. last week that "we do have to be concerned about the possibility that a terrorist organisation may seek to exploit our refugee resettlement process," according to WND.

He said the possibility "is true of this country, that's true of every other country that accepts refugees."

Fox News reported that more than 102,000 Syrians have become legal-permanent residents in the U.S. since 2012. The U.S. revoked more than 122,000 foreign-national visas since 2001 including 9,500 due to threat of terrorism.

In a House Oversight Committee meeting on Thursday, Michele Thoren Bond, assistant secretary for the Bureau of Consular Affairs, told Republican Rep. Jason Chaffetz that officials did not know what happened to those individuals whose visas were revoked because of terrorism, according to Fox News.

"You don't have a clue do you?" Chaffetz asked Bond.

"The sheer number of people arriving on all kinds of visas and with green cards, and possibly U.S. citizenship, makes it impossible for our counterterrorism authorities to keep track of them all, much less prevent them from carrying out attacks or belatedly try to deport them," said Jessica Vaughan of the Center for Immigration Studies.

Meanwhile, U.S. President Barack Obama described the Republicans' expression of concern over the matter as fear-mongering. He likened Syrian migrants to Jews persecuted under Germany's Nazi government.

"In the Syrian refugee of today, we should see the Jewish refugee of World War 2," Obama said.

Relatedly, two terror suspects were arrested in Salzburg, Austria, last week for having links to the Paris attacks last month. They were found hiding in a refugee centre in Salzburg.

"Two people who arrived from the Middle East were arrested at the weekend in accommodation for refugees on suspicion of belonging to a terrorist organisation," Robert Holzleitner, a spokesman for the Salzburg prosecutor's office, said, according to Reuters.

The San Bernardino shooting also had two Islamic radicals as suspects.