"Using nuclear technology is an obligation and a national demand; any going back in the nuclear field is the same as losing the country's independence - which will have a very high price," Ayatollah Ali Khamenei said on state television.
"Any retreat will open the way for a series of endless pressure and never-ending back-down. The way [chosen by Iran] is with no return," said the all-powerful supreme leader.
But Iran also announced that it had resumed negotiations with Russia on a compromise proposal aimed at resolving the long-running crisis over its nuclear work, which the United States claims is a cover for a weapons program.
Khamenei rejected accusations by the Western countries who have urged Iran to suspend sensitive nuclear activities due to the Islamic regime's years of concealment.
"The main motive of the United States and the countries that follow in its footsteps are to prevent Iran from obtaining its right to master advanced technology," he said.
"In their propaganda, they say Iran has concealed its nuclear activities; this is irrational, as no countries announce to the public the research activities until they succeed ... like what the Islamic Republic of Iran has done."
His comments came a day after Iran's arch foe the United States renewed economic sanctions first imposed in 1995, as US President George W. Bush piled on the pressure ahead of the Security Council meeting expected this week.
Iran's hardline President Mahmud Ahmadinejad defended his country's nuclear program and said in a fiery speech "on this path, retreating by even an iota is meaningless", amid chants of "Death to America" by the crowd.
"We had the revolution in order not to listen ... So I'm telling you [Westerners], be angry at the Iranian people and you will die of this anger," he added.
On a visit to Indonesia US Secretary of State Condoleezza Rice called for the UN Security Council to send Iran a "very strong" message, hoping for a tough statement telling Iran to suspend uranium enrichment and return to negotiations.
Envoys of Britain, China, France, Russia and the United States - the five veto-wielding, permanent members of the council - met on Monday in New York and were expected to hold more talks on Tuesday.
It would be the fourth such meeting since International Atomic Energy Agency (IAEA) head Mohamed ElBaradei sent an assessment report on the Iranian nuclear program to the council on March 8.
But in an apparent bid to head off any punitive action by the 15-member council, Iran said that talks with Russian officials resumed on Monday on a proposal by Moscow to enrich Iranian uranium on Russian soil.
A close ally that is helping Iran build its first nuclear power station, Russia confirmed that it had held confidential talks on Tuesday in Moscow with Iran and cautioned the United States against raising its military presence in the strategic Caspian Sea region, bordering Iran.
The Security Council, unlike the IAEA, has the authority to impose punitive measures such as economic sanctions on countries that fail adequately to answer IAEA questions.
The IAEA wants Iran to halt sensitive uranium enrichment, which is used in nuclear fuel cycle but can also make the core of an atomic bomb.
A major buyer of Iranian oil and a permanent Security Council member, China said on Tuesday that the crisis over Iran's nuclear program can still be ended through diplomacy and urged Tehran to keep talking with Russia over Moscow's compromise proposal.
Russia has reportedly made clear in the talks that it wants to see the impasse resolved through diplomatic means using the capabilities of the IAEA.
"During the talks, the Russian side reiterated its attachment to diplomatic means for resolving the problem using the means of the IAEA," a spokesman for the Russian Security Council was quoted as saying.
"The consultations will continue," he added.
The Russian reports spoke only of meetings held on Tuesday in Moscow while earlier Tehran said that the talks had begun on Monday.
The Russian delegation in the negotiations was led by Igor Ivanov, the head of the Security Council, while the Iranian team was led by Ali Hosseini-Tash, the deputy secretary of Iran's Supreme National Security Council.
"Iran believes negotiation based on international agreements is the solution to get out of the present situation and accepts all constructive proposals in this respect," the Council spokesman Hossein Entezami said.
The latest crisis was sparked when Iran refused to comply with an IAEA demand to suspend the research activities on enrichment that it had resumed on January 10.
© 2006 Agence France-Presse
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