A U.S. drone aircraft fired on two leaders of a militant Somali organization tied to al-Qaeda, apparently wounding them, a senior U.S. military official familiar with the operation said Wednesday.

The strike last week against senior members of al-Shabab comes amid growing concern within the U.S. government that some leaders of the Islamist group are collaborating more closely with al-Qaeda to strike targets beyond Somalia, the military official said.

The airstrike makes Somalia at least the sixth country where the United States is using drone aircraft to conduct lethal attacks, joining Afghanistan, Pakistan, Libya, Iraq and Yemen. And it comes as the CIA is expected to begin flying armed drones over Yemen in its hunt for al-Qaeda operatives.

Al-Shabab has battled Somalia’s tenuous government for several years. In recent months, U.S. officials have picked up intelligence that senior members of the group have expanded their ambitions beyond attacks in Somalia.

“They have become somewhat emboldened of late, and, as a result, we have become more focused on inhibiting their activities,” the official said.“They were planning operations outside of Somalia.”

Both of the al-Shabab leaders targeted in the attack had “direct ties” to American-born cleric Anwar al-Aulaqi, the military official said. Aulaqi escaped a U.S. drone strike in Yemen in May.

The White House declined Wednesday night to respond to questions about the attack.

But Obama administration officials have made repeated references to al-Shabab in recent weeks, indicating that the group has expanded its aims and its operations. In a speech Wednesday unveiling the administration’s new counterterrorism strategy, senior White House aide John O. Brennan included Somalia among the countries where the administration has placed a new focus on al-Qaeda affiliates.

“As the al-Qaeda core has weakened under our unyielding pressure, it has looked increasingly to these other groups and individuals to take up its cause, including its goal of striking the United States,” said Brennan, Obama’s chief counterterrorism adviser. “From the territory it controls in Somalia,” he said, “al-Shabab continues to call for strikes against the United States.”

And earlier this month, in a hearing to confirm him as Obama’s new defense secretary, CIA Director Leon Panetta told senators that the agency had intelligence on al-Shabab “that indicates that they, too, are looking at targets beyond Somalia.” Panetta said al-Qaeda had moved some of its operations to “nodes” in Yemen, Somalia and North Africa. The CIA, he said, was working with the U.S. Joint Special Operations Command in those areas “to try to develop counterterrorism.”

The Special Operations Command carried out last week’s Somalia strike, the military official said, and it has been flying remotely piloted planes over Yemen for much of the past year. It has taken the lead in operations in Yemen, where Aulaqi, a senior figure in al-Qaeda in the Arabian Peninsula, is based.

U.S. aircraft and Special Operations commandos have carried out other attacks in Somalia against militants linked to al-Qaeda, but the strike last week appears to have been one of the first U.S. drone attacks in Somalia.

It was not immediately clear what kind of unmanned aircraft was used in the attack or where the drone originated.

The airstrike appears to be one piece of a larger effort to step up offensive action against al-Shabab militants with ties to al-Qaeda in Somalia. Somali media have reported numerous rumors in recent months of U.S. airstrikes on militant camps.

On April 6, an al-Shabab commander was reported to have been killed by an airstrike in Dhobley, a border town in southern Somalia, according to the Web site Long War Journal.

This month, Fazul Abdullah Mohammed, the alleged architect of the 1998 U.S. Embassy bombings in East Africa, was killed in a shootout in the Somali capital, Mogadishu, Somali officials said. Mohammed was a founder of al-Shabab and was considered the most-wanted man in East Africa.

The United States conducted a DNA analysis to confirm Mohammed’s demise, a U.S. official said. Secretary of State Hillary Rodham Clinton described it as “a significant blow to al-Qaeda, its extremist allies and its operations in East Africa.”

In last week’s attack, local officials told the Associated Press that military aircraft struck a convoy carrying the militants as they drove along the coastline of the southern port city of Kismaayo late Thursday. Other local residents told journalists that an air attack had taken place on a militant camp near Kismaayo, an insurgent stronghold. Several residents were quoted as saying that more than one explosion had occurred over a period of several hours and that they thought that at least helicopters had taken part in the attack.

An al-Shabab leader confirmed the airstrike and said two militants were wounded. Abdirashid Mohamed Hidig, Somalia’s deputy defense minister, said the attack was a coordinated operation that killed “many” foreign fighters.

“I have their names, but I don’t want to release them,” he told the AP.

In the early days of the Obama administration, officials became concerned about Somali extremists and debated whether al-Shabab, despite some ties to al-Qaeda,posed a threat to the United States or was primarily focused on Somalia. Some administration and intelligence officials said the group’s objectives remained domestic and argued against any preemptive strike on its camps.

Over the past year, al-Shabab has focused more openly outside Somalia in its statements and targets. In July, the group carried out suicide bombings in Kampala, Uganda, that killed 76 people, including one American. Uganda is one of the countries providing troops to a peacekeeping force that protects the U.S.-backed government in Somalia.

In August, the Justice Department charged 14 people in this country with providing support to al-Shabab. Attorney General Eric H. Holder Jr. said that the indictments “shed further light on a deadly pipeline that has routed funding and fighters to al-Shabab from cities across the United States.”

Staff researcher Julie Tate contributed to this report.