WASHINGTON - Despite threats of retaliation from Pakistani militants, senior administration officials said Monday that the United States intended to step up its use of drones to strike militants in Pakistan's tribal areas and might extend them to a different sanctuary deeper inside the country.
On Sunday, a senior Taliban leader vowed to unleash two suicide attacks a week like one on Saturday in Pakistan's capital, Islamabad, unless the Central Intelligence Agency stopped firing missiles at militants. Pakistani officials have expressed concerns that the missile strikes from remotely piloted aircraft fuel more violence in the country, and some American officials say they are also concerned about some aspects of the drone strikes.
But as Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, and Richard C. Holbrooke, the special envoy to the region, arrived in Islamabad on Monday, the administration officials said the plan to intensify missile strikes underscored President Obama's goal to "disrupt, dismantle and defeat" Al Qaeda in Pakistan and Afghanistan, as well as to strike at other militant groups allied with Al Qaeda.
Officials are also proposing to broaden the missile strikes to Baluchistan, south of the tribal areas, unless Pakistan manages to reduce the incursion of militants there.
Influential American lawmakers have voiced support for the administration's position.
Senator Carl Levin, a Michigan Democrat who heads the Armed Services Committee, acknowledged last week that "the price is very heavy" when missile strikes mistakenly kill civilians, but he said the strikes were "an extremely effective tool."
The plans have met strong resistance from Pakistani officials and have also worried some former American officials and some analysts, who say that strikes create greater risks of civilian casualties and could further destabilize the nuclear-armed nation.
"You will be complicating and compounding anti-Americanism here," said Talat Masood, a retired Pakistani general and military analyst in Islamabad. "How can you be an ally and at the same time be targeted?"
Some American experts say a crucial change in aerial warfare, in which American forces are now often stalking individuals rather than tanks and other large armaments as in past wars, has raised new legal issues.
A. John Radsan, who worked as a C.I.A. lawyer from 2002 to 2004, argued in a recent scholarly article he wrote with Richard W. Murphy, a fellow law professor, that the United States should follow the lead of the Israeli Supreme Court and require an investigation of "targeted killings" by the C.I.A. to control the practice.
While the notion of remote-control killing may seem chilling, military experts say the drones, which can transmit live video for nearly a day at a time, typically supply the weapons targeting officers with enough information to avoid civilian casualties.
Marc Garlasco, a former military targeting official who now works for Human Rights Watch, the international advocacy group, said the drones had helped limit civilian casualties in Afghanistan and Iraq, where the Air Force uses them to attack people laying roadside bombs and to attack other insurgents.
But in trying to take advantage of what can be fleeting chances to kill top Taliban and Qaeda leaders in Pakistan, the C.I.A. faces a much more difficult task, especially if it follows the targets into more populated areas.
"When you're operating under very short time frames, like the C.I.A. is in Pakistan, you are exponentially increasing the risk of killing noncombatants," Mr. Garlasco said.
In Pakistan, the extensive missile strikes have been limited to the tribal areas, and authorities say they have killed 9 of the top 20 Qaeda leaders. American officials say the missile strikes have forced some Taliban and Qaeda leaders to flee south toward Quetta, a city in the province of Baluchistan, which abuts the parts of southern Afghanistan where recent fighting has been the fiercest.
One of the prized attributes of the drones - the Cessna-size Predators and their larger and more heavily armed cousins called the Reapers - is that they can linger over an area day after day, sending back video that can be used to build a "pattern of life" analysis.
Some experts have compared them to mini-satellites that can monitor a suspected terrorist compound for weeks, watching where the people go and with whom they interact, to help confirm that the right people are being singled out for attack.
Experts say the drones also carry laser-guided weapons with small warheads that are precise enough to kill a group of people in a street without damaging nearby buildings.
Like the military services, the C.I.A. uses computer software to assess possible collateral damage, and the fusing on the bombs can be adjusted to limit their impact.
But in Pakistan and Afghanistan, it can also be hard to evaluate tips about the locations of Taliban or Qaeda leaders if there are no troops nearby to help check them out.
While the Air Force operates its drones from military bases in the United States, the C.I.A. controls its fleet of Predators and Reapers from its headquarters in Langley, Va.
The final preparations for strikes in Pakistan take place in a crowded room lined with video screens, where C.I.A. officers work at phone banks and National Security Agency personnel monitor electronic chatter, according to former C.I.A. officials.
The intelligence officers watch scratchy video captured by the drones, which always fly in pairs above potential targets.
According to the former officials, it is generally the head of the C.I.A.'s clandestine service or his deputy who gives the final approval for a strike. The decision about what type of weapon to use depends on the target, according to one former senior intelligence official.
Top national security leaders have approved lists of people who can be attacked, officials say, and the lawyers determine whether each attack can be justified under international law.
Mark Mazzetti and Elisabeth Bumiller contributed reporting.