Iran's all-powerful supreme leader on Thursday rejected a US offer to negotiate one-on-one on Tehran's nuclear ambitions, ruling out such contacts so long as Washington keeps up its threats against the Islamic republic.

"I am not a diplomat but a revolutionary and I speak frankly," Ayatollah Ali Khamenei told air force commanders in remarks published on his website. "You (Americans) are pointing the gun at Iran and say either negotiate or we will shoot."

"Some rejoice at the offer of negotiations ... (but) negotiations will not solve anything," he said, adding that those in Iran who prefer to risk "American domination" by negotiating with Washington would be dealt with.

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

His stance appeared to contradict that of Foreign Minister Ali Akbar Salehi who said on Monday that he detected signs the United States was rethinking its approach towards Tehran.

Khamenei said: "Iran will not accept to negotiate with he who threatens us with pressure," in reference to a list sanctions adopted by Washington to coerce Iran into curbing its nuclear programme.

"The offer of talks is meaningful when the other side shows goodwill," he said.

The remarks come at a time when Tehran and six world powers are preparing to resume stalled talks over Iran's nuclear programme in the Kazakh city of Almaty on February 26.

Iran and the P5+1 group of the United States, China, Russia, Britain, France and Germany held three rounds of talks last year, the last of which ended in stalemate in June in Moscow.

Calls to roll back its atomic work were rebuffed by Tehran, which demanded world powers scale back sanctions which have caused pain for its struggling economy.

"Does imposing, in your own words, crippling sanctions show goodwill or hostility?" Khamenei said on Thursday, responding to a new offer of bilateral talks proposed by US Vice President Joe Biden last week.

Biden said at the Munich Security Conference that Washington was open to direct talks with Iran to resolve the nuclear issue provided "the Iranian leadership, supreme leader, is serious."

"We have made it clear at the outset that... we would be prepared to meet bilaterally with the Iranian leadership," Biden said.

"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 an array of issues, including Tehran's nuclear ambitions which the West and Israel suspect are aimed at military objectives, despite Iran's repeated denials.

Washington broke off relations with Iran in 1980 in the aftermath of the storming of the US embassy in Tehran that 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."

Iran is also being pressured by the UN's atomic watchdog agency to allow broader access to its nuclear facilities in a bid to resolve outstanding issues over the Islamic republic's past atomic activities.

A team from the agency is expected in Tehran on February 13.