WASHINGTON/SANAA — When a barrage of drone-fired missiles hit al-Qaida cells in Yemen in mid-April and killed dozens of militants, the results were strikingly different from a mistaken U.S. attack on a Yemeni wedding convoy just four months earlier.

But even though the drones apparently found their targets this time, they were still blamed for a number of civilian deaths.

Former CIA director Michael Hayden said Washington's new calculus should be to look at the value of each strike in terms of whether it is worth "alienating friends and feeding the al-Qaida narrative."

Although President Barack Obama promised greater transparency in a speech at the National Defense University, U.S. lawmakers are increasingly critical of the secrecy surrounding the operations.

Despite some spectacular drone hits that took out militant leaders in places such as Yemen and Pakistan, there are growing concerns in Washington that the net effect of the targeted-killing program may be counterproductive. "Collateral damage" is seen as an al-Qaida recruiting tool that undercuts the main rationale for the drone campaign — to make Americans safer.

"It's never a good idea to make more enemies than you get rid of," a former U.S. national security official said.

In his speech on May 23 last year, Obama defended the drone program as effective while promising to narrow its scope, but he is showing no sign of relinquishing what has become his counterterrorism weapon of choice since he took office in 2009.

Drones are spreading to new areas as U.S. operations hone in on al-Qaida affiliates in far-flung places like Somalia and in Nigeria, where American forces are helping search for more than 200 girls kidnapped by the Islamist group Boko Haram.

"Here we are, a year later, asking 'what has really changed?' " said University of Notre Dame law professor Mary Ellen O'Connell, a leading expert on extrajudicial killings who has testified before U.S. congressional committees. "The drones are still flying and the president still sees the attractiveness of this cold and antiseptic means of killing."

Obama's vision of shifting control of the drone program from the shadowy paramilitary arm of the Central Intelligence Agency to the more publicly accountable Pentagon is moving at what one national security source described as a "glacial pace."

Apart from bureaucratic impediments, the main obstacle may be concern about civilian casualties among top lawmakers such as Dianne Feinstein, chair of the Senate Intelligence Committee, who see the CIA as better at killing with accuracy.

The Pentagon's Joint Special Operations Command is widely believed to have been behind the Dec. 12 drone strike in a remote part of Yemen that hit a convoy later identified as a wedding procession, killing 15 people. An official U.S. inquiry was launched but no findings have been released.

The number of allegedly bungled military strikes in Yemen led to a suspension of the Pentagon's drone operations there earlier this year, while the CIA, which has its own fleet, continued drone operations, a national security source said.

Obama, in last year's speech, said drone strikes would be barred unless there was "near certainty" that no civilians would be hit and the administration says every precaution is taken to avoid killing the innocent.

The New America Foundation, which compiles drone casualties, put the number of militants killed in U.S. strikes in Yemen this year at 79 in addition to four civilians.

"Our forces go to extraordinary lengths to avoid civilian casualties," said Caitlin Hayden, spokeswoman for the White House National Security Council. "But when we believe that civilians may have been killed despite these efforts, we investigate thoroughly."

Washington has long argued that reports of hundreds of civilian deaths in the U.S. drone war are exaggerated, though in the absence of the government's own casualty counts it is all but impossible to verify the assertion.

There are clear signs that "collateral damage" feeds anti-American sentiment in the Muslim world and fuels sympathy for groups such as Yemen's al-Qaida in the Arabian Peninsula, which Washington sees as a threat to the U.S. homeland.

"We oppose drone attacks because more people are dying," said Mohamed al-Qawli, head of Yemen's National Organization for Drone Victims. "It is killing outside the law."

Qawli's brother Ali, a science teacher, was killed in 2013 when a taxi he and a nephew were riding in picked up some strangers. A missile obliterated the car. At least six suspected militants died, local sources said. The Yemeni government said Ali and his nephew were innocent civilians.

"My brother was completely charred. We identified him by his teeth," Qawli told Reuters. Afterwards, people in the area started listening to al-Qaida tapes and exchanged militant videos on mobile phones, Qawli said.