The comments came in an hourlong interview with The Associated Press on the first day of his visit to the United States to attend the annual general assembly of the United Nations this week.
He insisted that his government does not want an atomic bomb - something he has said in the past - and that Iran is only seeking peace and a nuclear-free world. He gave no indication of when Iran would resume talks on its nuclear program and said any anti-nuclear sanctions against his government would have no effect on his government's policies.
Appearing calm and self-assured, the Iranian president said he was pleased about the release of American hiker Sarah Shourd from a prison in Tehran, but said her two companions still in prison would have to prove their innocence on charges that they illegally crossed into the country.
"The United States' administrations ... must recognize that Iran is a big power," he said. "Having said that, we consider ourselves to be a human force and a cultural power and hence a friend of other nations. We have never sought to dominate others or to violate the rights of any other country.
"Those who insist on having hostilities with us, kill and destroy the option of friendship of us in the future, which is unfortunate because it is clear the future belongs to Iran and that emnities will be fruitless - and hence sanctions, too, will be ineffective."
He added: "If they were to be effective, I should not be sitting here right now."
Ahmadinejad asserted that international nuclear regulators had never found proof that Iran is pursuing an atomic bomb.
"We are not afraid of nuclear weapons. The point is that if we had in fact wanted to build a nuclear bomb, we are brave enough to say that we want it. But we never do that. We are saying that the arsenal of nuclear bombs (worldwide) have to be destroyed as well," he said.
The U.S. accuses Iran of hiding plans to build a nuclear bomb; Iran denies that and says it's working only toward building nuclear power plants.
On the case of the American hikers, Ahmadinejad said, `We're very glad that that lady was released. (Due) to the humanitarian perspective of the Islamic Republic chose to adopt on the subject, she was released on bail. And we hope that the other two will soon be able to prove and provide evidence to the court that they had no ill intention in crossing the border, so that their release can also be secured."
Tying the case to Iran's assertion that some of its citizens are being held unjustly in the United States, he said, "It certainly does not give us joy when we see people in prison, wherever in the world that may be, and even when we think of prisoners here."
His answers were translated from Farsi by an Iranian translator, but Ahmadinejad appeared to be following the questions in English and occasionally corrected his interpreter.
Asked about retired FBI employee Robert Levinson, who disappeared during a trip to Iran in 2007, Ahmadinejad appeared to suggest that he may have been Iran for suspicious reasons.
"The U.S. government had informed us of this situation and we announced that we are unaware of it. We agreed to establish a joint information committee to trace ... his whereabouts.
"Of course if it becomes clear what his goal was, or if he was indeed on a mission, then perhaps specific assistance can be given," the Iranian said. "For example, if he had plans to visit with a group or an individual or go to another country, he would be easier to trace in that instance."
Levinson was last seen on Iran's Kish island in March 2007 where he had gone to seek information on cigarette smuggling for a client of his security firm. He had been an FBI agent in New York and Florida before retiring in 1998. He has not been seen since. Iran says it has no information on him.