The U.S. Air Force understands what caused the crash of one of its unmanned spy drones over Iran late last year and continues to use that type of drone, Air Force Chief of Staff General Norton Schwartz said.

Schwartz, in an interview with Reuters last week, said the drones are providing intelligence, surveillance and reconnaissance (ISR) data to military commanders.

U.S. officials reject Iran's claim that it brought down the stealthy RQ-170 Sentinel built by Lockheed Martin Corp, but remain tight-lipped about what caused the crash.

U.S. officials told Reuters last month that they were investigating a combination of pilot error and technical malfunction as possible causes.

Schwartz declined comment on the outcome of the investigation, but said the Air Force now understood what caused the crash and was continuing to use the rest of the service's RQ-170 spy planes to provide data.

"The key thing is that it's an ISR system that we use to provide capabilities to the combatant commanders and we'll continue to do so," Schwartz said in an interview.

He also said the crash had not raised concerns about work on the classified spy plane by Lockheed Martin, the Pentagon's No. 1 supplier.

The Air Force operates "more than a handful" of the unmanned spy planes, and continues to fly them out of Kandahar, Afghanistan, according to one senior defense official and a former senior official, neither of whom could speak publicly given the sensitive nature of the program.

The plane lost in Iran was on a mission for the CIA, but the Air Force also uses the planes for other surveillance missions over Afghanistan, the officials said.

The radar-evading aircraft measures over 40 feet from wing tip to wing tip, and carries a full-motion video sensor that was used last year by U.S. intelligence to monitor al Qaeda leader Osama bin Laden's compound in Pakistan ahead of the raid that killed him. It features special coatings that make it nearly invisible to enemy radar.

Lockheed has declined comment on the Iran incident or what might have gone wrong with the plane, which came out of the company's secretive Skunk Works facility in southern California.

Company officials have referred all questions about the incident to the Air Force, which first acknowledged the existence of the drone in December 2009.

Iran announced on December 4 it had downed the spy plane in the eastern part of the country, near Afghanistan. It subsequently showed an image of the apparently intact plane on television and said it was close to cracking its technological secrets.

The loss of the plane sparked some concerns that sophisticated technology could fall into the hands of China or other countries that are actively developing their own unmanned planes. The main concern about technology Iran could pilfer from the drone centers on the special coatings on the craft's surface.

The computers onboard the drone are believed to have been heavily encrypted and its sensors were not the most sophisticated tools in the U.S. arsenal.