The unmanned surveillance plane lost by the United States in Iran was a stealth aircraft being used for secret missions by the CIA, U.S. officials said Monday.

The officials said Iran’s military appears to be in possession of one of the more sensitive surveillance platforms in the CIA’s fleet, an aircraft that was shaped and designed to evade enemy defenses.

The mission of the downed drone remains unclear. Iran, a longtime adversary of the United States, is believed by U.S. intelligence agencies to be pursuing the development of a nuclear weapon and is also accused of providing support to anti-coalition elements in both Iraq and Afghanistan.

The RQ-170 has been used by the CIA for highly sensitive missions into other nations’ airspace, including months of surveillance of the compound in Pakistan in which Osama bin Laden was hiding before he was killed in a May raid by Special Operations forces.

A CIA spokeswoman declined to comment on whether the drone was being flown by the agency. A Pentagon spokesman, George Little, also declined to comment.

The disclosure that the drone apparently recovered by Iran was being flown by the CIA comes after previous signals from U.S. officials that had created the impression that the plane was being flown by the U.S. military on a more mundane mission over Afghanistan and had simply strayed into Iranian territory.

A statement issued by the International Security Assistance Force in Afghanistan on Sunday said that the downed drone “may be a U.S. unarmed reconnaissance aircraft that had been flying a mission over western Afghanistan late last week. The operators of the UAV lost control of the aircraft and had been working to determine its status.”

U.S. officials insisted Monday that the statement was technically accurate, noting that it did not explicitly assert that the aircraft was being operated by ISAF or the U.S. military. Instead, the language of the statement was ambiguous about the drone’s ownership and mission. The CIA’s role in operating the plane was first reported Monday by NBC News.

Some U.S. officials began to cast doubt on the ISAF statement almost as soon as it was released. Current and former U.S. defense officials said it was unlikely that the military would be using a highly sophisticated stealth aircraft — one presumably in relatively short supply — for surveillance operations over western Afghanistan.

The statement’s suggestion that U.S. pilots lost control of the aircraft was accurate, according to U.S. officials who have disputed claims by Iran that its defense forces downed the aircraft, or that it had been felled by a sophisticated cyberattack.

Although the ISAF statement said that the drone was flying “a mission over western Afghanistan,” CIA drones are generally used to conduct surveillance as well as strikes beyond that country’s borders, most notably in Pakistan. The agency’s aircraft do take off from and land at air bases in Afghanistan, however. There is no indication that the drone lost in Iran was armed.

The RQ-170 has special coatings and a batwing shape designed to help it penetrate other nations’ air defenses undetected. The existence of the aircraft, which is made by Lockheed Martin, has been known since 2009, when a model was photographed at the main U.S. airfield in Kandahar, Afghanistan.

Staff writer Greg Jaffe contributed to this article.