Iran's Supreme Leader has warned any Israeli attack would be answered with a "lightning" response and suggested Iran's nuclear programme cannot be curtailed by Western sanctions.
The remarks by Ayatollah Ali Khamenei echoed previous hard-line positions by Iran, but take on added resonance amid talks with the US and five other world powers.
Western leaders hope for a diplomatic accord that would ease concerns about Iran's nuclear ambitions, while Israeli officials say they leave all options open to try to derail Iran's uranium enrichment.
The West fears Iran could one day produce weapons-grade material.
Khamenei called the claims of a secret weapons programme "lies" and repeated Iran's statements that it only seeks reactors for energy and medical research.
Khamenei put Israel on notice that any military action would bring swift consequences.
"Should they take any wrong step, any inappropriate move, it will fall on their heads like lightning," he warned in a speech marking the 23rd anniversary of the death of his predecessor, Ayatollah Ruhollah Khomeini, the leader of the 1979 Islamic Revolution.
Many military analysts say air strikes alone are unlikely to seriously set back Iran's uranium enrichment and could touch off a wider conflict in the Gulf, which is the route for about one fifth of the world's oil.
Instead, the US and Europe have imposed tighter sanctions targeting Iran's oil exports and its ability to conduct international banking.
"The obstacles enemies are creating in our path won't have any effect. Sanctions are ineffective. Sanctions can't stop the Iranian nation from moving forward," Khamenei said at Khomeini's mausoleum south of Tehran.