The CIA’s use of surveillance drones over Iran reflects a growing belief within the Obama administration that covert action and carefully choreographed economic pressure may be the only means of coercing Iran to abandon its nuclear ambitions, current and former U.S. officials say.

The administration’s shift toward a more confrontational approach — one that also includes increased arms sales to Iran’s potential rivals in the Middle East as well as bellicose statements by U.S. officials and key allies — suggests deepening pessimism about the prospects for a dialogue with Iran’s leaders, the officials say.

The administration’s evolving strategy includes expanded use of remote-controlled stealth aircraft, such as the one that came down in eastern Iran last week, as well as other covert efforts targeting Iran’s nuclear program, according to U.S. government officials and Western diplomats, who spoke on the condition of anonymity to discuss intelligence-gathering efforts.

The U.S. officials said the stealth drone was part of a fleet of secret aircraft that the CIA has used for several years in an escalating espionage campaign targeting Iran’s nuclear facilities.

As those efforts have surged, the White House also has boosted sales of bunker-busting munitions, fighter jets and other military hardware to Persian Gulf states as well as to Israel, building on long-running efforts to boost the military capabilities of key U.S. allies in the region, the officials say.

Underscoring the implied military threat, Defense Secretary Leon E. Panetta last week cited contingency plans for “a wide range of military options” to be used against Iran if necessary. He expressed the administration’s “determination to prevent Iran from developing nuclear weapons,” a phrase that suggested an intention to stop the Islamic republic from obtaining the technological building blocks of nuclear arms. Previous White House statements have vowed only to prevent Iran from acquiring nuclear bombs.

The sharpened tone comes against a backdrop of increased diplomatic efforts to ratchet up the economic pain for the Iranian regime, as Washington enlists European and Asian allies in coordinated efforts to choke Iran’s economy.

But while endorsing the increased use of sanctions, U.S. officials also are growing increasingly aware of the limits of such measures. A congressional study released this week suggested that Iran has managed to limit the damage to its economy from international sanctions — in part because of immense profits gained from near-record oil prices in recent years. And the study warns that harsher sanctions targeting Iran’s petroleum and banking industries could drive oil prices still higher.

“The easy stuff has been done already,” said a senior administration official involved in strategy toward Iran. “The choices now are much harder.”

The more-robust measures stand in contrast with the administration’s early optimism that it could draw Iran’s ruling clerics into negotiations on curbing their country’s nuclear program. Although insisting that the door remains open to talks, administration officials see little evidence that top Iranian officials are interested in engaging, or capable of doing so.

“There’s greater skepticism now,” said Ray Takeyh, a former State Department official who advised the administration on Iran policy in 2009, when President Obama famously made direct appeals to Iran in an attempt to improve relations. Since Iran’s rebuff of numerous public and private overtures, the administration’s goal is to “press Iran further and isolate Iran further,” Takeyh said.

Current and former U.S. officials say the administration is ramping up its covert efforts inside Iran, even as the White House is seeking a thaw in bilateral relations.

The officials say the RQ-170 Sentinel drone that went down over Iran was part of a fleet of secret aircraft that enabled the CIA to carry out dozens of high-altitude surveillance flights deep into Iranian territory without being detected.

A former senior Defense Department official said the stealth drone flights had been underway for “at least four years,” The aircraft, built by Lockheed Martin, is best known for its role in surveilling the compound in Pakistan where Osama bin Laden was killed. “But it wasn’t only being flown in Pakistan,” the former official said.

The CIA is thought to have a dozen or so of the batwing-shaped, radar-evading aircraft, which are capable of being fitted with different “sensor payloads,” meaning they can be equipped to capture a range of intelligence material, including high-resolution images, radiation measurements and air samples.

U.S. officials have described the loss of the aircraft in Iran as a setback but not a fatal blow to the stealth drone program. “It was never a matter of whether we were going to lose one but when,” the former official said, indicating that the CIA had used technologies that it could afford to have exposed.

Among the main concerns is that Iran could use an intact aircraft to examine the vulnerabilities in stealth technology and take countermeasures with its air defense systems. Another is that China or other adversaries could help Iran extract data from the drone that would reveal its flight history, surveillance targets and other capabilities.

It is unclear whether the drone was programmed to destroy such data in the event of a malfunction. Nor is there agreement on how the aircraft went down. U.S. officials have dismissed Iranian assertions that it was shot or brought down by a cyberattack. Instead, explanations have focused on potential technical failures. The aircraft cover great distances and depend on satellite links. A lost connection or other malfunction could cause them to drift off course and crash when they run out of fuel or room to fly.

Officials said the stealth flights have contributed significantly to improved intelligence on Iran’s nuclear efforts.

Iran’s nuclear program has long been a focus of satellite flights and collection from human sources. But the drone flights have enabled the CIA to fill in substantial gaps, making it difficult for Iran to use windows between satellite passes to move material or conduct tests.

“It’s such a powerful tool to be able to keep eyes on a location for an extended period,” said a former senior U.S. intelligence official. “If you can park something up there, you can get to a situation where somebody can’t do anything without being detected.”

The emphasis of covert measures over diplomacy is unsettling to some former U.S. officials who praised the White House’s earlier attempts at rapprochement with Iran. Greg Thielmann, a former State Department official, said he suspected that the administration was pulling back on its diplomacy because of intensifying pressure from the political right.

“Considering the stakes involved, I can’t accept the idea that we should accept failure and move on to other options,” said Thielmann, who is a senior fellow at the Washington-based Arms Control Association.

Officially, the Obama administration espouses what White House officials call a “dual-track strategy” of seeking diplomatic engagement with Iran while steadily applying increasing economic and political pressure. On Wednesday, after Iranian authorities blocked access to a Web-based “Virtual Embassy” where ordinary Iranians could access uncensored information about the United States, the State Department released a statement underscoring the U.S. preference for negotiations.

“The United States remains steadfast in our commitment to a dialogue with the Iranian people,” the statement read.