CIA drone attacks in Pakistan killed at least 581 militants last year, according to independent estimates. The number of those militants noteworthy enough to appear on a U.S. list of most-wanted terrorists: two.

Despite a major escalation in the number of unmanned Predator strikes being carried out under the Obama administration, data from government and independent sources indicate that the number of high-ranking militants being killed as a result has either slipped or barely increased.

Even more generous counts - which indicate that the CIA killed as many as 13 "high-value targets" - suggest that the drone program is hitting senior operatives only a fraction of the time.

After a year in which the CIA carried out a record 118 drone strikes, costing more than $1 million apiece, the results have raised questions about the purpose and parameters of the campaign.

Senior Pakistani officials recently asked the Obama administration to put new restraints on a targeted-killing program that the government in Islamabad has secretly authorized for years.

The CIA is increasingly killing "mere foot soldiers," a senior Pakistani official said, adding that the issue has come up in discussions in Washington involving President Asif Ali Zardari. The official said Pakistan has pressed the Americans "to find better targets, do it more sparingly and be a little less gung-ho."

Experts who track the strikes closely said a program that began with intermittent lethal attacks on al-Qaeda leaders has evolved into a campaign that seems primarily focused on lower-level fighters. Peter Bergen, a director at the New America Foundation, said data on the strikes indicate that 94 percent of those killed are lower-level militants.

"I think it's hard to make the case that the 94 percent cohort threaten the United States in some way," Bergen said. "There's been very little focus on that question from a human rights perspective. Targeted killings are about leaders - it shouldn't be a blanket dispensation."

Even former CIA officials who describe the drone program as essential said they have noted how infrequently they recognized the names of those killed during the barrage of strikes in the past year.

The CIA declined to comment on a program that the agency refuses to acknowledge publicly. But U.S. officials familiar with drone operations said the strikes are hitting important al-Qaeda operatives and are critical to keeping the United States safe.

"This effort has evolved because our intelligence has improved greatly over the years, and we're able to identify not just senior terrorists, but also al-Qaeda foot soldiers who are planning attacks on our homeland and our troops in Afghanistan," said a U.S. official, who spoke on the condition of anonymity to discuss the classified program.

"We would be remiss if we didn't go after people who have American blood on their hands," the official said. "To use a military analogy, if you're only going after the generals, you're likely to be run over by tanks."