The strikes killed 581 militants last year but the toll included only two of the top terrorists on a U.S. list, The Washington Post reported Monday, citing the New America Foundation.
The National Counterterrorism Center said the two on the list were Sheik Saeed al-Masri, al-Qaida's No. 3 leader, and Ahmed Mohammed Hamed Ali, who was chief of paramilitary operations in Afghanistan.
Data from government and independent sources showed the number of high-ranking militants killed in the heightened strikes, ordered under President Barack Obama's administration, has either come down or remained about the same, the Post said.
The results come a year after the U.S. Central Intelligence Agency launched a record 118 drone strikes at a cost of more than $1 million each and raise questions about their purpose, the report said.
One Pakistani official was quoted as saying the CIA effort was only killing "mere foot soldiers." He said his government has urged Americans "to find better targets, do it more sparingly and be a little less gung-ho."
Peter Bergen, director at the New America Foundation, told the Post data on the strikes showed 94 percent of them killed lower-level militants.
"Targeted killings are about leaders -- it shouldn't be a blanket dispensation," he said.
The Post said the CIA declined comment as it has not acknowledged the program publicly. But U.S. officials familiar with the operation say the strikes are hitting important al-Qaida operatives and are critical to keeping the United States safe.