Iranian President Mahmoud Ahmadinejad warned here Tuesday that a new round of UN sanctions against Tehran would close the door on diplomatic engagement with the United States.

"The path to that (improved ties) will be shut," Ahmadinejad told reporters, adding that it would mean a "reversal to the (former US president George W.) Bush era" of confrontation rather than the engagement US President Barack Obama seeks.

Ahmadinejad, who was to leave for Iran later in the day, was clearly in a combative mood after on Monday blasting the United States, at a UN conference on the Non-Proliferation Treaty (NPT), for blocking other nations' access to peaceful nuclear energy.

Obama came to office last year saying he wanted to talk with Iran over US concerns that the Islamic Republic is using a civilian nuclear program to hide the development of atomic weapons.

But the US president also advocates increasing pressure on Iran with UN sanctions if Tehran turns down offers to negotiate.

Washington has led the drive for three rounds of UN Security Council economic sanctions on Iran since December 2006 to get it to stop enriching uranium, which can be used to make atomic bombs.

If the Council were to adopt a fourth round, "the relationship between Iran and the United States will never improve again," Ahmadinejad told a press conference on the sidelines of the NPT meeting.

He said he was concerned "that the opportunity presented as a result of Mr. Obama coming to office to reform America's image in the world, an image that was shaped by taking a position against other nations, would be lost."

Ahmadinejad said that despite this Iran would not withdraw from the NPT, the treaty governing the world's fight to stop the spread of nuclear weapons, nor stop cooperating with the UN watchdog International Atomic Energy Agency.

"My presence here means that we want the NPT to be revised, that it becomes a fair system and that we remain an active member of the IAEA," Ahmadinejad said.

He said Iran did not want sanctions imposed against it but would thrive if they were.

"The heaviest political pressures have been imposed on us since the revolution... We do not embrace or welcome any of these resolutions or sanctions," he said.

But he said: "Experience has proven that sanctions cannot stop the Iranian nation. The Iranian nation is able to withstand all the pressures brought against it by the United States and its allies."

Iran has "managed to turn that into opportunity for progress... Iran today is far more advanced than 30 years ago," he said.

"We feel and think that the US government will be damaged more than us by those sanctions" as "sanctions in a free trade world are a broke deal" that will hurt the United States, Ahmadinejad said.

If Washington "insists on falling from a cliff, there's nothing that can be done," the Iranian President said.

Ahmadinejad at the NPT conference Monday had charged that Washington was threatening Iran with nuclear weapons, an interpretation of a US policy that does not rule out using the bomb against states in non-compliance with the NPT.

US Secretary of State Hillary Clinton on Monday had dismissed the Iranian leader's charges as "wild accusations" in her speech to the opening session of the three-week review conference.

Ahmadinejad said Tuesday that Clinton had come with an "angry posture, threatens Iran, shouts at Iran. This is impossible."

He challenged her to say what was wrong with his comments on the nuclear issue.

"The NPT has been weak, failed and needs major revision" as it is dominated by nuclear weapons states which block other countries from using atomic energy for peaceful purposes such as generating electricity, Ahmadinejad said.