Iran's Supreme Leader Ayatollah Ali Khamenei has hailed Iran's "great victory" over its nuclear programme.
Mr Khamenei praised President Mahmoud Ahmadinejad's handling of the issue.
Last week, the UN nuclear watchdog said Iran was being more transparent, but had not given "credible assurances" that it was not building a bomb.
On Monday, the agency heard that Iran may have continued secret work on nuclear weapons after 2003, the date US intelligence suggested the work ceased.
"The Iranian nation has honestly and seriously achieved a great victory"
Tehran has dismissed the allegations as "forgeries".
The permanent members of the UN Security Council - the US, UK, China, France and Russia - are meeting in Washington to discuss the possibility of imposing further sanctions on Iran over its disputed atomic programme.
Western countries suspect Iran aims to produce a nuclear bomb. Tehran insists its programme is aimed purely at generating electricity.
Praise
"One example of an advance by the Islamic system has been the nuclear issue, in which the Iranian nation has honestly and seriously achieved a great victory," Mr Khamenei said on Tuesday.
Iran's key nuclear sites
"Those people who used to say Iran's nuclear activity must be dismantled are now saying we are ready to accept your advances, on condition that it will not continue indefinitely.
"This is a great advance that would not have been realised except with perseverance," he said.
He went on to praise President Ahmadinejad's "outstanding" handling of the stand-off with the West.
Under Iran's system of government, the supreme leader had the final say in major policy matters.
In a rare public gesture last month, Mr Khamenei overruled the president over the implementation of a gas sector bill.
The fact that he decided to go public on the issue was interpreted as a signal that he wanted to convey he was not happy with the president.
Mr Ahmadinejad has said no amount of UN sanctions will deter Tehran from its nuclear path.
"If they want to continue with that path of sanctions, we will not be harmed. They can issue resolutions for 100 years," he said in a televised interview on Saturday.
A US National Intelligence Estimate released last December said Tehran had frozen its atomic programme in 2003.
But documents presented to the International Atomic Energy Agency (IAEA) suggest the work continued.
The material was presented to the agency's 35-nation board by the IAEA's head of safeguards, Olli Heinonen, in a closed-door meeting on Monday.
Simon Smith, Britain's ambassador to the IAEA, said material presented to the IAEA in Vienna came from multiple sources and included designs for a nuclear warhead, plus information on how it would perform and how it would fit onto a missile.
"Certainly some of the dates that we were talking about ... went beyond 2003," he said.
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