WASHINGTON - Since the suicide bombing that took the lives of seven Americans in Afghanistan on Dec. 30, the Central Intelligence Agency has struck back against militants in Pakistan with the most intensive series of missile strikes from drone aircraft since the covert program began.
Beginning the day after the attack on a C.I.A. base in Khost, Afghanistan, the agency has carried out 11 strikes that have killed about 90 people suspected of being militants, according to Pakistani news reports, which make almost no mention of civilian casualties. The assault has included strikes on a mud fortress in North Waziristan on Jan. 6 that killed 17 people and a volley of missiles on a compound in South Waziristan last Sunday that killed at least 20.
"For the C.I.A., there is certainly an element of wanting to show that they can hit back," said Bill Roggio, editor of The Long War Journal, an online publication that tracks the C.I.A.'s drone campaign. Mr. Roggio, as well as Pakistani and American intelligence officials, said many of the recent strikes had focused on the Pakistani Taliban and its leader, Hakimullah Mehsud, who claimed responsibility for the Khost bombing.
The Khost attack cost the agency dearly, taking the lives of the most experienced analysts of Al Qaeda whose intelligence helped guide the drone attacks. Yet the agency has responded by redoubling its assault. Drone strikes have come roughly every other day this month, up from about once a week last year and the most furious pace since the drone campaign began in earnest in the summer of 2008.
Pakistan's announcement on Thursday that its army would delay any new offensives against militants in North Waziristan for 6 to 12 months is likely to increase American reliance on the drone strikes, administration and counterterrorism officials said. By next year, the C.I.A. is expected to more than double its fleet of the latest Reaper aircraft - bigger, faster and more heavily armed than the older Predators - to 14 from 6, an Obama administration official said.
Even before the Khost attack, White House officials had made it clear to Dennis C. Blair, the director of national intelligence, and Leon E. Panetta, the C.I.A. director, that they expected significant results from the drone strikes in reducing the threat from Al Qaeda and the Pakistani Taliban, according to an administration official and a former senior C.I.A. official with close ties to the White House.
These concerns only heightened after the attempted Dec. 25 bombing of a Detroit-bound airliner. While that plot involved a Nigerian man sent by a Qaeda offshoot in Yemen, intelligence officials say they believe that Al Qaeda's top leaders in Pakistan have called on affiliates to carry out attacks against the West. "There's huge pressure from the White House on Blair and Panetta," said the former C.I.A. official, who spoke on condition of anonymity out of concern about angering the White House. "The feeling is, the clock is ticking."
After the Khost bombing, intelligence officials vowed that they would retaliate. One angry senior American intelligence official said the C.I.A. would "avenge" the Khost attack. "Some very bad people will eventually have a very bad day," the official said at the time, speaking on the condition he not be identified describing a classified program.
Today, officials deny that vengeance is driving the increased attacks, though one called the drone strikes "the purest form of self-defense."
Officials point to other factors. For one, Pakistan recently dropped restrictions on the drone program it had requested last fall to accompany a ground offensive against militants in South Waziristan. And tips on the whereabouts of extremists ebb and flow unpredictably.
A C.I.A. spokesman, Paul Gimigliano, declined to comment on the drone strikes. But he said, "The agency's counterterrorism operations - lawful, aggressive, precise and effective - continue without pause."
The strikes, carried out from a secret base in Pakistan and controlled by satellite link from C.I.A. headquarters in Virginia, have been expanded by President Obama and praised by both parties in Congress as a potent weapon against terrorism that puts no American lives at risk. That calculation must be revised in light of the Khost bombing, which revealed the critical presence of C.I.A. officers in dangerous territory to direct the strikes.
Some legal scholars have questioned the legitimacy under international law of killings by a civilian agency in a country where the United States is not officially at war. This month, the American Civil Liberties Union filed a request under the Freedom of Information Act for government documents revealing procedures for approving targets and legal justifications for the killings.
Critics have contended that collateral civilian deaths are too high a price to pay. Pakistani officials have periodically denounced the strikes as a violation of their nation's sovereignty, even as they have provided a launching base for the drones.
The increase in drone attacks has caused panic among rank-and-file militants, particularly in North Waziristan, where some now avoid using private vehicles, according to Pakistani intelligence and security officials. Fewer foreign extremists are now in Miram Shah, North Waziristan's capital, which was previously awash with them, said local tribesmen and security officials.
Despite the consensus in Washington behind the drone program, some experts are dissenters. John Arquilla, a professor of defense analysis at the Naval Postgraduate School who frequently advises the military, said, "The more the drone campaign works, the more it fails - as increased attacks only make the Pakistanis angrier at the collateral damage and sustained violation of their sovereignty."
If the United States expands the drone strikes beyond the lawless tribal areas to neighboring Baluchistan, as is under discussion, the backlash "might even spark a social revolution in Pakistan," Mr. Arquilla said.
So far the reaction in Pakistan to the increased drone strikes has been muted. Last week, Prime Minister Yousaf Raza Gilani of Pakistan told Richard C. Holbrooke, the administration's senior diplomat for Afghanistan and Pakistan, that the drones undermined the larger war effort. But the issue was not at the top of the agenda as it was a year ago.
Hasan Askari Rizvi, a military analyst in Lahore, said public opposition had been declining because the campaign was viewed as a success. Yet one Pakistani general, who supports the drone strikes as a tactic for keeping militants off balance, questioned the long-term impact.
"Has the situation stabilized in the past two years?" asked the general, speaking on condition of anonymity. "Are the tribal areas more stable?" Yes, he said, Baitullah Mehsud, founder of the Pakistani Taliban, was killed by a missile last August. "But he's been replaced and the number of fighters is increasing," the general said.
Sabrina Tavernise contributed reporting from Islamabad, Pakistan, and Ismail Khan from Peshawar.