The Federal Bureau of Investigation (FBI) is finding itself at odds with the Obama administration as it warned Congress for a second time this year that admitting thousands of mostly Muslim migrants into the US is a "highly dubious venture, fraught with risks that terrorist fighters could slip in posing as refugees."

Testifying before Congress on Wednesday, FBI Director James Comey dismissed the contention made by State Secretary John Kerry that the mostly Syrian migrants pose no threat to US national security, WND reported.

Comey told members of the House Homeland Security Committee that US authorities could not possibly conduct background checks on all the incoming refugees from Syria because there are no "data points" that would show their past activities.

Kerry has been trying to calm jittery Americans who fear that members of the Islamic State (ISIS) jihadist group could suddenly emerge from the refugee ranks and start causing big trouble. Kerry said this is unlikely since the refugees would be the "most highly scrutinised" and "most rigorously vetted" of all travelers when they start pouring into the country.

Kerry made the assurance even as the ISIS has already made known that it intends to exploit the refugee flow to send operatives to the US and other Western countries.

"The challenge we're all talking about is that, we can only query against that (data) which we have collected," Comey said. "And so if someone has never made a ripple in the pond in Syria in a way that would get their identity or their interest reflected in our database, we can query our database until the cows come home but there would be nothing to show up because we have no record on it."

Comey said it would have been better if the refugees were from Iraq since the US had a "database" of information there "because of our country's work there for a decade."

Meanwhile as the debate rages over the issue, the Obama administration said it has already brought in more than 1,850 Syrian refugees. Some 97 percent of the migrants are Muslims.

Thousands of Christians being persecuted in the Middle East are also seeking to resettle in the US, but the Obama administration has shown little sympathy and has slammed the door on their desire to find refuge in the US, reports said.

Ironically, President Obama said he plans to bring in 10,000 more Muslim refugees in the US over the next year.

The situation in made even more surreal as 14 Democrat senators want Obama to bring in at least 65,000 more Syrian Muslim refugees.

Moreover, this number is still not large enough for refugee lobbying groups such as the US Conference of Catholic Bishops, the Lutheran Immigration and Refugee Service and the International Rescue Committee. They have called on the US government to accept at least 100,000 Syrian refugees.