There has been no credible evidence to prove that members of the Islamic State are planning to use the porous southern border to enter the United States from Mexico, Department of Homeland Security officials told Congress on Wednesday. Instead, administration officials are more concerned about ISIS jihadists entering the U.S. through commercial airline flights.

Although a senior Homeland Security (DHS) official, Francis Taylor, told senators that ISIL supporters are known to be plotting ways to infiltrate the United States through the border, another DHS spokesperson claimed that there was "no credible intelligence to suggest that there is an active plot by ISIL to attempt to cross the southern border," Washington Free Beacon reported.

"There have been Twitter, social media exchanges among ISIL adherents across the globe speaking about that as a possibility," Taylor told Sen. John McCain (R., Ariz.) in response to a question about "recent reports on Twitter and Facebook of messages that would urge infiltration into the U.S. across our southwestern border."

The Homeland Security testimony cast in doubt the border scenarios cited by conservative lawmakers such as Sen. Ted Cruz of Texas, Fox News reported.

"We don't have any credible information, that we are aware of, of known or suspected terrorists coming across the border," Jennifer Lasley, a senior official in the Department of Homeland Security's intelligence and analysis office, told the House Homeland Security border security subcommittee.

"Certainly any infiltration across our border would be a threat," Taylor said, explaining that border security agents are working to tighten measures that would prevent this from taking place.

"I'm satisfied we have the intelligence and the capability on our border that would prevent that activity," Taylor, DHS undersecretary for intelligence, said.

However, citing recent videos featuring activist James O'Keefe's stunt to expose the lack of security present on America's northern border, McCain questioned why border agents had been unable to stop him.

When Taylor could not provide an answer, McCain said, "You can't answer it because they weren't there to stop him," according to Washington Free Beacon.

"The fact is there are thousands of people who are coming across our border who are undetected and not identified, and for you to sit there and tell me that we have the capability or now have the proper protections of our southwest border, particularly in light of the urgings over Facebook and Twitter [by ISIL] for people to come across our southwestern border, is a great concern to the citizens of my state."

There is little evidence to prove that ISIL militants or other terror actors would be stopped if they attempt to cross the border, McCain said.

"I don't think there's any doubt, I don't see when you look at ISIS and the growth and influence of ISIS that it would be logical [to claim they would be stopped], as they're saying on Facebook and Twitter, to come across our southwest border because they can get across," he said.

While numerous calls to heighten security along the border in the wake of the ISIS threat have been made by Cruz, McCain, and other lawmakers, officials told Congress that the administration of President Barack Obama is more concerned with jihadist fighters holding western passports and entering the U.S. legally on commercial airline flights.

"The number of known watch-listed persons we are encountering on the Southwest border is minimal compared to commercial aviation," said John Wagner, assistant commissioner in the U.S. Customs and Border Patrol's Office of Field Operations. "We're talking tens versus thousands."

Meanwhile, about 100 American citizens are reported to be fighting with the Islamic State in Iraq and Syria, with at least three Americans getting killed, including one in a suicide bombing, according to U.S. intelligence.

"Our concern remains that these individuals, if radicalized, could return to their home countries or to the U.S., and use their newly acquired skills to carry out attacks," Lasley testified.

"Although we currently have no credible information to indicate that [the Islamic State] is planning to attack the homeland," she said, "we remain concerned in the long term."