Ayatollah Ali Khamenei also poured scorn on US "staggering to and fro"
Iran's supreme leader has insisted it will continue its nuclear activities for civilian purposes only and will not manufacture nuclear weapons.
"No wise nation would be interested in making a nuclear weapon today. They are against rational thought," said Ayatollah Ali Khamenei in a speech.
The comments come a day after the UN's atomic watchdog urged from Iran "full disclosure" about its atomic work.
Its report said alleged research into warheads was "of serious concern".
Some western powers accuse Iran of seeking to develop nuclear arms.
Iran has always denied such charges but has refused to suspend some nuclear work which could have military application, prompting three rounds of sanctions imposed by the UN.
"[Iran] is after peacefully using nuclear energy for the purposes of daily life and it will follow this path," Mr Khamenei said in the speech to mark the 19th anniversary of the death of the leader of the Islamic revolution, Ayatollah Ruhollah Khomeini.
"Contrary to what the enemy wishes, it will reach it [nuclear energy] with full strength," he added.
The speech also poured criticism on the US administration, saying President George W Bush and his advisers acted like "the mentally ill".
"Sometimes they threaten, sometimes they order assassinations... and sometimes they ask for help - it's like mad people staggering to and fro," he said.
European Union foreign policy chief Javier Solana is planning to travel to Tehran soon to submit a new package of incentives for Iran to stop its most sensitive nuclear work.
Iran has said its nuclear activities, including uranium enrichment, is sanctioned under the Nuclear Non-Proliferation Treaty of which it is a signatory.
International Atomic Energy Agency is currently holding a week-long board of directors meeting.
On Monday the agency announced that Syria was to allow its inspectors to visit the country to investigate allegations that it was building a nuclear reactor.