WASHINGTON (Reuters) - The Pentagon has found no evidence to back Irans claims it shot down drone surveillance aircraft in the Gulf, officials said on Monday, acknowledging some of the spy planes had crashed due to mechanical failure. Irans semi-official Fars news agency quoted a senior Revolutionary Guards commander on Sunday as saying Iran had shot down two unmanned Western reconnaissance drone aircraft in the Gulf. Commander Amir Ali Hajizadeh, head of the elite forces aerospace unit, did not say when the alleged incidents took place or produce proof they had happened. Pentagon spokesman Colonel Dave Lapan said there were no recent reports that would corroborate what the Revolutionary Guard said about unmanned aerial vehicles. Another U.S. military official, speaking on condition of anonymity, told Reuters that the last incident he was aware of involved a drone aircraft crashing in the Gulf in early 2009. Like others before it, the crash was due to mechanical failure and the aircraft landed in international waters, the official said. We have had cases in which UAVs have gone down in the Gulf ... but I dont have any indication (of a case) where a UAV has been taken down by hostile fire in the Gulf, the official said. Were still researching whether there has ever been a case, the official said, adding that such an event could not yet be definitively ruled out. Iran is at odds with major powers over its nuclear activities, which the United States and its allies suspect are aimed at producing a nuclear weapon. Tehran has denied the allegations and said it wants only to generate electricity. The United States and Israel, Irans main foes, have not ruled out military action if diplomacy failed to end the nuclear row. Iran has dismissed reports of possible U.S. or Israeli plans for a military strike, but said it would respond by attacking U.S. interests and Israel in the event of such an assault. Analysts say Tehran could retaliate by launching hit-and-run strikes in the Gulf and by closing the Strait of Hormuz. About 40 percent of traded oil leaves the Gulf region through the strategic waterway.