Despite Ahmadinejad's insistence that there is no evidence of Iran working towards a nuclear weapon, U.N. Secretary General Ban Ki-moon called on Iran earlier in the conference to "clarify the doubts and concerns" that Iran is stockpiling and producing nuclear weapons.
The head of the International Atomic Energy Agency said at the U.N. he is "unable to confirm that all nuclear material is in peaceful activities because Iran has not provided the necessary cooperation."
"Iran's nuclear program is clouded, both because there are serious questions about the peaceful nature of that program and because they have not granted the IAEA the transparency necessary for them to address these questions," said Jacqueline Shire, a senior analyst and nuclear expert at the Institute for Science and International Security.
Ahmadinejad's visit comes at a sensitive time as the United States and its allies are pushing for another round of tough sanctions on Tehran.
The U.S. would like to see unanimous support on the United Nations Security Council for tough sanctions to isolate Iran, but Tehran has sought to drive a wedge between them and other countries on the council who are wary of supporting additional sanctions at this time, namely non-veto members Brazil and Turkey. Lebanon, whose government includes members of the Iranian-backed militant group Hezbollah, is not expected to vote in favor of a sanctions resolution. Lebanon holds the body's rotating presidency in May, making a vote on sanctions unlikely until June.
Both Brazil and Turkey have said they believe there is still time to negotiate with Iran. Turkey recently suggested it could help in a proposed deal that would send some of Iran's nuclear fuel abroad for enrichment before it is returned for the Tehran Research Reactor, thereby denying Iran the opportunity to use it in a bomb. That proposal was put on the table last October, but has yet to be accepted by Iran.
Brazil's president will visit Tehran later this month to try to persuade Ahmadinejad to negotiate. Brazil has been lobbied heavily by Secretary of State Hillary Clinton who traveled there in March to discuss the matter. She'll meet again today with Brazil's foreign minister.
The United States has modest goals for the NPT review conference, held every five years. U.S. officials say they hope most countries will agree to back strengthening the IAEA so that the nuclear inspections body can then step up its pressure on Iran.
"The Review Conference operates by consensus, which makes the adoption of bold, ground-breaking final documents unlikely," said ISIS' Shire.
Critical for the conference's success will be the support of Egypt, which leads the 188-members Non-Aligned Movement. Egypt, however, has said it wants to discuss one of the world's worst-kept secrets: Israel's undeclared nuclear weapons program.
The Obama administration has made nuclear security a cornerstone of its foreign policy. Last year Obama called for a nuclear-free world during a speech in Prague. This year, Obama traveled to the Czech capital again to sign a new nuclear arms reduction treaty with Russia. Last month Obama convened an unprecedented gathering of world leaders in Washington to discuss preventing the proliferation of nuclear material.
The U.S. hopes to avoid the failure of the last NPT review conference in 2005, which fell apart in large part due to concerns from many countries that the U.S. had not done enough to make good on a previous pledge to reduce its nuclear arsenal.
ABC News' Devin Dwyer contributed to this report.