Sanaa, Yemen (CNN) -- Three suspected air strikes hit militant targets in southern Yemen Monday night and Tuesday morning, killing at least nine people believed to be linked to al Qaeda, Yemeni security officials said.
A senior U.S. official confirmed to CNN Tuesday that a strike was carried out by a mix of drones and manned aircraft under the control of the Defense Department, not the CIA. Both drones and manned aircraft fired.
The post-attack assessment by U.S. officials was still underway Tuesday. A determination had yet to be made on how many people were killed, and who was killed.
A second U.S. official called the attack "very complex."
Three Yemeni security officials said missiles struck in the Abyan province near areas that have been taken over by Ansaar al-Sharia, a militant group with links to al Qaeda.
The militant group took over large parts of the province in May after government forces evacuated a number of security posts and military bases. The group announced Abyan as an Islamic emirate and is calling for the implementation of Sharia law.
Hundreds of troops and militant fighters have been killed in the government's efforts to clear the province from the hands of the militants, according to Yemen's defense ministry.
Some residents in the area said they saw the aircraft in the attacks.
"The U.S. aircraft was flying in surprisingly low altitude Monday night. We were scared and expected a night of explosions and blood," said Mousa Abdul Kadoos Abu Ali, a resident of Lowder district.
U.S. officials rarely discuss the drone program, though privately they have said the covert strikes are legal and an effective tactic in the fight against extremists. But President Barack Obama did speak about the controversial use of drone attacks Monday.
"Our ability to respect the sovereignty of other countries and to limit our incursions into somebody else's territory is enhanced by the fact that we are able to pinpoint-strike an al Qaeda operative in a place where the capacities of that military in that country may not be able to get them," the president said during a Google+ video chat room interview Monday.
He added that "drones have not caused a huge number of civilian casualties." He gave no indication that the U.S. policy of ordering drone strikes would change, at least as long as a terrorist threat remains.
"Al Qaeda has been really weakened, but we've still got a little more work to do," Obama said. "And we've got to make sure we are using all our capacities in order to deal with it."
In September, a CIA-operated drone attack in Yemen killed American cleric Anwar al-Awlaki, the external operations commander and chief recruiter of English-speaking militants for the al Qaeda in the Arabic Peninsula (AQAP).
While the death may reduce AQAP's ability to plan attacks, the group still poses a danger, and "remains the node most likely to attempt transnational attacks," according to the annual U.S. intelligence community's threat assessment, released Tuesday.
CNN's Barbara Starr contributed to this report.