And she also accused Britain of "dismantling" the Anglo-US-led coalition in Iraq by pulling troops out of Basra too soon.
The all-party group of MPs say Debra Cagan, Deputy Assistant Secretary for Coalition Affairs to Defence Secretary Robert Gates, made the comments this month.
The six MPs were taken aback by the hardline approach of the Pentagon and in particular Ms Cagan, one of Mr Bush's foreign policy advisers.
She made it clear that although the US had no plans to attack Iran, it did not rule out doing so if the Iranians ignored warnings not to develop a nuclear bomb.
It was her tone when they met her on September 11 that shocked them most.
The MPs say that at one point she said: "In any case, I hate all Iranians."
Although it was an aside, it was not out of keeping with her general demeanour.
"She seemed more keen on saying she didn't like Iranians than that the US had no plans to attack Iran," said one MP. "She did say there were no plans for an attack but the tone did not fit the words."
Another MP said: "I formed the impression that some in America are looking for an excuse to attack Iran. It was very alarming."
Tory Stuart Graham, who was on the ten-day trip, would not discuss Ms Cagan but said: "It was very sobering to hear from the horse's mouth how the US sees the situation."
Ms Cagan, whose job involves keeping the coalition in Iraq together, also criticised Britain for pulling out troops.
"She said if we leave the south of Iraq, the Iranians will take it over," said one MP.
Another said: "She is very forceful and some of my colleagues were intimidated by her muscular style."
The MPs also saw Henry Worcester, Deputy Director of the Office of Iranian Affairs, who said he favoured talks with Iran.
The Pentagon denied Ms Cagan said she "hated" Iranians.
"She doesn't speak that way," said an official.
But when The Mail on Sunday spoke to four of the six MPs, three confirmed privately that she made the remark and one declined to comment. The other two could not be contacted.
© 2007 The Daily Mail