General Hossein Moghadam, the Guard's former deputy chief, was speaking after film footage showed strings of freshly dug graves in the south of the country.
They were close to the site of war graves for the dead of the long war between Iran and Saddam Hussein's Iraq, which devastated the region in the 1980s.
"The mass graves that used to be for burying Saddam's soldiers have now been prepared again for US soldiers, and this is the reason for digging this big number of graves," Gen Moghadam told the Associated Press, which obtained the footage.
The warning is unlikely to be more than symbolic. No-one expects a land invasion, should the White House authorise a strike on nuclear facilities, while Iran has so far suggested counter-action is most likely to be aimed at American allies in the Gulf and Western bases there.
Gen Moghadam's claims might be a sign that President Mahmoud Ahmadinejad, who is increasingly reliant on the Revolutionary Guard for political backing, is feeling the heat of international diplomatic pressure over his aggressive posture on Iran's uranium enrichment programme.
Washington has made full use of the diplomatic victory it won in forcing through a new United Nations sanctions package, with the unexpected support of both Russia and China, in June.
Iranian businessmen both inside and outside the country say the economy is suffering, while President Ahmadinejad's many enemies from within the ranks of his own conservative faction in the leadership are frequently outspoken on his domestic record across the board.
Last week Admiral Mike Mullen, chairman of the American joint chiefs of staffs, kept up President Barack Obama's double-handed approach, saying that he had prepared an attack plan focusing on Iran's nuclear plants while at the same time accepting he was unwilling to use it.
Both sides are open about the brinkmanship threats of force involve, with the wider Middle East fearing it will be sucked into open conflict. Gen Moghadam went on to say: "If the US decides to take a pre-emptive action and attack Iran, Iran will have no choice but to strike the American bases in the region.
"The heavy costs of such a war will not be just on the Islamic Republic of Iran. America and other countries should accept that this would be the start of an extensive war in the region."