Islamic State extremists have discussed infiltrating the U.S. through its southern border with Mexico, a U.S. official said.

Francis Taylor, undersecretary for intelligence and analysis at the Department of Homeland Security, told a Senate committee Wednesday that the Sunni militants have been tracked discussing the idea on social media sites such as Twitter Inc.

"There have been Twitter and social media exchanges among ISIL adherents across the globe speaking about that as a possibility," Taylor said. Islamic State is also known by the acronyms ISIL and ISIS. Referring to the 1,933-mile (3,110- km) boundary with Mexico, Taylor said he was "satisfied that we have the intelligence and the capability at our border that would prevent that activity."