Earlier on Monday, Iran asked for a technical review of the plan designed to restrain its potential for making a nuclear bomb.
The UN plan requires Iran to ship about 70 per cent of its uranium abroad for conversion into fuel before being sent back to a Tehran reactor monitored by the UN nuclear watchdog.
The US and other Western countries are concerned that Iran may be enriching uranium for use in nuclear weapons, but Tehran insists its programme is strictly for research and energy production.
On Monday, the head of the UN nuclear watchdog urged Iran to immediately clarify its response to the proposal, which is backed by the US, Russia, China, Britain, France and Germany.
Mohamed ElBaradei, the head of International Atomic Energy Agency (IAEA), said "a number of questions and allegations relevant to the nature" of Iran's programme remained, and called for confidence building measures on all sides.
"The issue at stake remains that of mutual guarantees amongst the parties," he said in his final address to the UN General Assembly before ending his tenure later this year.
"This is a unique and fleeting opportunity to reverse course from confrontation to co-operation and should, therefore, not be missed."
ElBaradei added that "trust and confidence-building are an incremental process that requires focusing on the big picture and a willingness to take risks for peace".
Last week the IAEA said it had received an "initial response" from Iran to the deal.
Iran's foreign minister said the country was not rejecting the offer but Iranian officials have sent mixed signals on the plan.
Iran has indicated interest in purchasing ready-made uranium from abroad rather than shipping its low-enriched uranium for further enrichment.
Ali Asghar Soltanieh, Iran's chief envoy to the IAEA, told The Associated Press news agency that Iran wants "to buy the fuel from any supplier".
Some experts say Iran has little reason to trust the West and for that reason may be in no hurry to cut a deal.
"Iran believes time is on their side for now," said Mustafa Alani, a regional analyst at the Gulf Research Centre in Dubai.