US national-security adviser Michael O'Hanlon suggests that top commander Adm. William Fallon abruptly resigned because he had cold feet.

According to O'Hanlon, Fallon's sudden decision to end his 42-year career in the Navy has caught Washington off guard.

"I can't clearly see what the reason would be for why he resigned," said O'Hanlon who is a senior fellow at the Brookings Institution think tank in Washington.

"Obviously, he said a couple of things that maybe were a little off-key from what the administration was otherwise saying -- the implication that we shouldn't be threatening for against Iran, for example," AFP quoted him as saying.

O'Hanlon claimed that the reason the 63-year-old resigned may have been that he simply 'lost heart' out of sheer exhaustion.

"Although Fallon oversees the Middle East region, the success that General Petraeus has had in Iraq may overshadow his ability to be effective in the job," explained the military expert.

Pundits strongly believe that the top military official's vocal opposition to the Bush administration's gung-ho stance on Iran is what led to his departure.

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