Al-Nusra Front, an aggressive 2-year-old jihadi faction fighting to overthrow the Assad regime, "does have aspirations for attacks on the homeland," Director of National Intelligence James Clapper told the Senate Intelligence Committee.
He did not discuss al-Nusra's capabilities or provide evidence of its wish to attack on U.S. soil.
Washington designates the group as a terrorist organization.
Clapper said Yemen's al-Qaida in the Arabian Peninsula -- the most active al-Qaida branch, or "franchise" -- was more capable than al-Nusra of carrying out a U.S. attack.
But he warned of a fast rise in militant Islamist fighters in Syria.
"The attraction of these foreign fighters is very, very worrisome," Clapper said in remarks quoted by Britain's Guardian newspaper.
He said some 26,000 fighters of an estimated 75,000 to 110,000 rebels battling the regime of President Bashar Assad were "extremists," and about 7,000 of them were not from Syria but from some 50 other countries, including "many" from Europe.