A reduction in U.S. drone strikes in Pakistan in recent months reflects success in targeting senior al Qaeda operatives and forcing militants "underground," a U.S. official said on Friday.
The United States suspended strikes by the unmanned aircraft in Pakistani borderlands for nearly two months late last year, partly to ease anger over a November 26 NATO air attack that killed 24 Pakistani soldiers and led Pakistan to close supply routes to U.S.-led troops in Afghanistan.
Strikes in the border area of North Waziristan resumed in January, but the rate of attacks has been scaled back so far this year, U.S. officials said.
Pakistan's parliament on Thursday unanimously approved recommendations on the country's ties with the United States, including a demand to end the strikes.
Some U.S. officials say sensitivities in Pakistan have been factors considered when the United States decided whether to launch strikes, but others say a lack of targets has been a key reason behind the reduction of attacks.
"What's important is not the number or pace of strikes, but their effectiveness," a U.S. official said on condition of anonymity.
"This year already an (al Qaeda) external-ops planner and another key Pakistani military ally have been taken off the battlefield," the official said.
"Another way to look at the number of strikes is to see this as the result of sustained and effective run of aggressive counter terrorism operations that have steadily degraded al Qaeda and its allies over the past several years."
Militant targets killed in strikes between late September and mid-October included a son of a blind Egyptian cleric serving a life sentence in the United States for plotting to attack New York City landmarks; and individuals Washington says were a subcommander in the al Qaeda-linked Haqqani network and two al Qaeda planners.
"From the end of September through October, al Qaeda and its militant allies experienced a series of significant losses," a senior U.S. official said.
"Within a matter of days, four significant targets were removed from the battlefield. After such successes, it is natural that important targets would go into even deeper hiding and it would take time to find them."
Bruce Riedel, a former CIA officer who is now a foreign policy expert at Washington's Brookings Institution, said the U.S. administration was also fully aware of Pakistani concerns.
"It's ... clear the administration knows the drone war is deeply unpopular in Pakistan and it needs to use them with greater care," he said.
"Now that the parliament formally has demanded they stop completely, the next drone attack sets up a showdown," he said. "Pakistan's finance minister visits Washington next week, the first cabinet visitor since last year. Will the drones fly?"
NATO has been seeking to persuade Pakistan to reopen its border crossings with Afghanistan, the closure of which has forced the alliance to use a costlier northern distribution route to get supplies to its troops.
The Pakistan government has yet to decide how to respond to parliament's recommendations. In a speech to parliament on Thursday, Pakistani Prime Minster Yusuf Raza Gilani did not say whether the NATO supply routes would be reopened.