"The Islamic Republic of Iran considers retreat over the nuclear issue ... as breaking the country's independence, which will impose huge costs on the Iranian nation," Khamenei said at a meeting with Iran's ambassadors and representatives in foreign countries.
"Any retreat at this point will bring an unending chain of pressures and further retreats. Therefore, this path is irreversible and the foreign policy establishment has to bravely defend Iran's right," he said.
Khamenei, who holds final say in all matters in Iran, ordered all diplomats abroad to defend Tehran’s nuclear program, saying that bowing to international pressure would force the Islamic Republic to gradually give up all its foreign policy goals.
President Mahmoud Ahmadinejad also vowed to resist mounting pressure over Iran’s nuclear program, saying that “no power“ can take nuclear fuel technology away from Tehran.
"Rest assured that the technology to produce nuclear fuel today is in the hands of the youth of this land, and no power can take it back from us," Ahmadinejad told a crowd of thousands in Gorgan, northern Iran.
The Iranian President also said that "major powers" were "trying to block the progress of other nations through bullying and harassment".
"They should know that by such pressures they cannot stop the Iranian people from continuing their course," he said.
The strong remarks by the Iranian leaders came hours before the UN Security Council was to start debating Iran’s nuclear case. The Council has the power to impose sanctions, but such a move would face tough resistance from permanent UN members, Russia and China.
The United States and the European Union accuse Iran of secretly developing an atomic weapons program. But Tehran denies their allegations, insisting that it has a right to work on a peaceful nuclear program as a signatory to the Nuclear Non-Proliferation Treaty.
The U.S. Secretary of State Condoleezza Rice said on Tuesday that she expected the Security Council to agree on a "very strong message" for Iran.
Meanwhile, Russian negotiators and Iranian officials held talks in Moscow on a Russian proposal aimed at ending Tehran’s standoff with the West.
The Russian Security Council said that its head, Secretary Igor Ivanov met Iranian negotiator Ali Hoseyni-Tash for closed-door talks.
Russia is trying to persuade Iran to move its uranium enrichment program to Russian territory.
The Iranians left Russia after the negotiations, with no announcement of any progress.