Iran supreme leader Ayatollah Ali Khamenei put a block on any direct talks with the US today.
A statement posted on the Ayatollah's website marked his first reaction to the idea of one-on-one talks, which have been floated for months and were reiterated last week by US Vice-President Joe Biden.
But any possible momentum depended on the ayatollah, who has the final say and has previously opposed direct negotiations with Washington.
"Talks will not solve any problems," he said.
"You are holding a gun against Iran saying you want to talk. The Iranian nation will not be frightened by the threats (of sanctions)."
His statement could spill over into negotiations in Kazakhstan later this month between Iran and a six-nation group comprising the permanent UN security council members plus Germany.
His reference to US sanctions suggested that Iranian envoys will stick to demands for relief from economic pressures before any nuclear concessions.
The US further tightened sanctions this week, extending measures which have already cut Iran's oil revenue by 45 per cent.
They call on countries that buy Iranian crude oil - mostly Asian nations including China and India - to set the money aside and require Iran use it to buy local products rather than get cash.
Three rounds of talks ended in stalemate last year, with Tehran pushing for a roll-back in Western sanctions in exchange for any concessions on its nuclear programme, but only facing steadily increasing censure from the United Nations as a result.