Al-Qaida still aims to strike inside the United States but home-grown or unaffiliated extremists now "pose an equally serious threat," FBI chief Robert Mueller warned U.S. lawmakers Thursday.

"Al Qaida and its affiliates are still committed to striking us in the United States," Mueller told a Senate Appropriations Subcommittee, pointing to plots to bomb New York City subways and the failed Christmas airline attack.

"Home-grown and lone-wolf extremists pose an equally serious threat," the Federal Bureau of Investigation director said, citing the shootings at the sprawling Fort Hood army base in Texas.

Experts have warned that a "lone wolf" — an extremist acting alone, without connections to an established network like Al-Qaida — may be the most difficult threat for authorities to thwart.

"We have also seen U.S.-born extremists plotting to commit terrorism overseas," such as Mumbai attacks planner David Headley, the U.S.-born son of a former Pakistani diplomat and American woman, said Mueller.

"These terrorist threats are diverse, far-reaching and ever-changing, and to combat these threats, the FBI must sustain our overseas contingency operations and engage our intelligence and law enforcement partners both here at home and abroad," he said.