Iran has warned the US to back its nuclear fuel swap offer or lose an "historic" chance of improving relations between the countries.
Washington has denounced the Iranian offer - brokered last week by Brazil and Turkey - as a ploy to avoid a new round of UN sanctions over its controversial nuclear programme, which the West fears is geared toward nuclear weapons.
President Mahmoud Ahmadinejad said: "There are people in the world who want to pit Mr Obama against the Iranian nation and bring him to the point of no return, where the path to his friendship with Iran will be blocked forever."
Iran proposed last week to send much of its low-enriched uranium to Turkey in return for nuclear fuel rods needed for a Tehran medical research reactor. The swap would diminish Iran's stockpile of low-enriched uranium that can possibly be used in making atomic bombs, if the uranium is enriched to a higher, weapons-grade level.
But the proposal did not deter the US, Russia, China, Britain and France - the five permanent Security Council members - from agreeing on a draft fourth set of sanctions against Iran for refusing to completely halt uranium enrichment, as demanded by the United Nations.
US Secretary of State Hillary Clinton said Tehran's offer is inadequate and does not address international concerns about its atomic ambitions.
"If they (US and its allies) are truthful when they say they seek co-operation ... they should accept this offer," Mr Ahmadinejad said. "But if they seek excuses, they should know that the path to any interaction will be closed."
"Mr Obama must know that this proposal is a historic opportunity ... (Mr Obama should) know that if this opportunity is lost, I doubt the Iranian nation will give a new chance to this gentleman in the future," he added.