US Defense Secretary Leon Panetta on Monday cast doubt on Iranian military claims that it started building a copy of a US surveillance drone captured last year.

Revolutionary Guards Commander Brigadier was quoted saying in Iranian media on Sunday that engineers were in the final stages of decoding data from the Sentinel aircraft, which came down in December near the Afghan border.

"It's obviously a classified program and I don't want to get into the particulars of that program," Panetta told reporters shortly before landing in Colombia. "But I think I can tell you based on my experience that I would seriously question their ability to do what they say they've done."

Iran said the unmanned aircraft was shot down but Washington disputes that and has said in the past that the security systems mean Iran is unlikely to get much valuable data from the Lockheed Martin Corp drone.

The RQ-170 Sentinel has been widely used since 2010 in Afghanistan and Pakistan. It played a role in the operation in which Osama bin Laden was killed last year, analysts say.

Iran has repeatedly displayed images of the American stealth plane through Iranian media, attempting to prove it skillfully captured the drone. The UAV is 5.4 meters in length, and has a wingspan of 26 meters, Iranian news agency Mehr reported. The aircraft is able to avoid detection by radar systems through "special materials that compose its exterior," according to the report.

Iran's downing of the American drone was an embarrassment for the Obama administration, which subsequently asked Iran to return the plane.

An Iranian defense official said recently that Tehran has received numerous requests for information on the craft and that China and Russia have shown most interest.

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