Government officials said Ahmadinejad would make new proposals to defuse suspicion over Tehran's nuclear weapons ambitions when he addresses the opening day of a three-day UN summit.
But he will be flying into a determined effort by US President George Bush to rally support for possible UN sanctions against the Islamic Republic for resuming sensitive work on uranium conversion.
"Iran with a nuclear weapon will be incredibly destabilising. And, therefore, we must work together to prevent them from having the wherewithal to develop a nuclear weapon," Bush said on Tuesday at the White House.
But the White House statement was followed by strong criticism of the US and the other 190-member states of the UN General Assembly by Secretary-General Kofi Annan.
He said the inability of members to agree on issues such as non-proliferation in drafting a blueprint for better global security was unacceptable.
"The big item missing [from the draft reform] is non-proliferation and disarmament (and) this is a real disgrace"
"The big item missing is non-proliferation and disarmament (and) this is a real disgrace," Annan said, referring to the draft document to be agreed on by heads of state during the three-day world summit.
The document was expected to reflect the original goals and ideals generally agreed upon by nations during the millennium summit in 2000.
Washington broadly welcomed the compromise reform package as a "very good beginning", though Undersecretary of State for Political Affairs Nicholas Burns conceded that it was not "a 100% victory".
Burns said more deliberation was needed on questions such as a definition of terrorism, an approach to monitoring human rights and enlargement of the Security Council.
The US has been highly critical of the UN and has pressed for deep reforms, particularly after a massive scandal broke over mismanagement of the former $100 billion oil-for-food programme for Iraq.