Iranian supreme leader Ayatollah Ali Khamenei rejected Thursday a US offer to negotiate one-on-one on Tehran's disputed nuclear programme so long as Washington continues to impose sanctions on Iran.

"You (Americans) want to negotiate when you are pointing the gun at Iran. The Iranian nation will not be intimidated by such actions," Khamenei told air force commanders, according to excerpts of his speech posted on his website.

"Some rejoice at the offer of negotiations ... (but) negotiations will not solve anything," he said.

The all-powerful Khamenei has final say on all key issues in the Islamic republic, including Iran's sensitive nuclear activities and foreign policy.

"I am not a diplomat but a revolutionary," he said. "The US policies in the Middle East have failed and the Americans are in need of a winning hand. That is bringing Iran to the negotiating table."

He made the remarks in response to an offer put forward by US Vice President Joe Biden to engage Iran directly to resolve the nuclear issue provided "the Iranian leadership, supreme leader, is serious."

Speaking at the Munich Security Conference last week, Biden said: "We have made it clear at the outset that... we would be prepared to meet bilaterally with the Iranian leadership.

"That offer stands, but it must be real and tangible, and there has to be an agenda that they're prepared to speak to. We are not just prepared to do it for the exercise," Biden said.

Khamenei's remarks came a day after the US tightened sanctions on Iran to further choke off its oil income.

The two foes are locked in a tense showdown over Tehran's nuclear ambitions, which the West and Israel suspect are aimed at military objectives, despite Iran's denial.

Washington broke off relations with Iran in 1980 in the aftermath of the storming of the US embassy in Tehran, which led to 52 American diplomats being taken hostage by Islamist students.

Since then, the US has been vilified by the Islamic republic as the "Great Satan".