"I think the Iraqis can defend their interests without the Iranians, thank you very much," Rice told reporters after a meeting with her Mexican counterpart, Patricia Espinosa.
A day earlier, Rice had told reporters on her plane to Mexico that Iraqi forces cannot yet defend Iraq by themselves, so Baghdad should accept the draft security pact that would allow U.S. troops to remain beyond the end of the year.
At a press conference on Thursday in the Mexican resort city of Puerto Vallarta, Rice said the draft pact that has been negotiated with Iraq's government both protected U.S. forces and "is totally respectful of Iraqi sovereignty."
Earlier on Thursday Iranian President Mahmoud Ahmadinejad was quoted by the state news agency IRNA as having warned the Iraqis that Washington would not keep its promises to them.
"They (the Americans) seek to prevent the establishment of a strong and honorable Iraq in order to continue their looting of the country," IRNA quoted Ahmadinejad as saying.
"Frankly, I don't take those comments very seriously," Rice said of Ahmadinejad's comments.
"That hasn't been the happiest relationship, ever," she said of Iran's ties with Iraq. The Iranians had been arming "special groups" of fighters in the south of Iraq, she said, referring to members of Shi'ite militant cells that the United States says Iran is supporting.
Those groups, Rice said, had been "killing innocent Iraqis."
Baghdad has exasperated Washington officials by calling for changes in what the Bush administration considered the "final draft" of the draft security pact, although the Iraqis have not spelled out publicly exactly what they want.
Rice said on Wednesday en route to Mexico that Baghdad had a strong interest in making sure U.S. forces remained there until Iraqi forces could defend Iraq, "but I don't think that anybody believes that they are capable of doing that alone right now."
The United States, which invaded Iraq in March 2003 to overthrow Saddam Hussein's government, has 155,000 troops in Iraq operating under a United Nations Security Council mandate that expires on Dec. 31.
The draft security pact would require U.S. forces to leave Iraq by the end of 2011 and also provides a mechanism for Iraqi courts to try American troops for serious crimes committed off duty. (Editing by Eric Walsh)
