Iran's supreme leader, Ayatollah Ali Khamenei, has threatened a "lightning response" to any Israeli attack and warned that sanctions would fail to curb his country's nuclear programme and only deepen its "hatred of the west".

The angry rhetoric is routine for Iran's clerical leadership but had particular significance as it came between rounds of negotiations between Tehran and the world's major powers over Iranian nuclear aspirations.

Western officials were hoping to hear conciliatory notes in the supreme leader's speech, delivered on the anniversary of the death of his predecessor, Ayatollah Ruhollah Khomeini, which is often the occasion of policy announcements by the Tehran leadership.

Khamenei's mildly positive language towards Barack Obama in March raised hopes for a diplomatic way out of the nuclear impasse but, with a third round of talks due in Moscow later this month, there was no encouragement for reconciliation in today's address.

"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. "The only effect these unilateral and multilateral sanctions have on the Iranian nation is that they deepen hatred and animosity toward the west in the heart of our people."

Turning towards the threat of an Israeli military strike against Iran's nuclear sites, the ayatollah warned: "Should they take any wrong step, any inappropriate move, it will fall on their heads like lightning."

Khamenei's comments come a week after the head of the Iranian atomic energy organisation, Fereydoun Abbasi-Davani, declared there was no need for Iran to stop production of 20% enriched uranium, something that had been sought by international negotiators at the last round of discussions in Baghdad last month. The Baghdad talks ended without significant progress. A six-nation group of negotiators from the US, China, UK, France, Germany, and Russia offered reactor fuel, nuclear safety support and parts for commercial airliners in return for a suspension of 20% enriched uranium production The Iranians said Tehran was willing to discuss 20% uranium but made no mention of suspension and demanded international recognition of its right to enrich uranium.

Western officials say the six UN security council resolutions calling for Iran to suspend enrichment – which can produce fuel for both power stations and nuclear weapons – could be reversed, but only as part of a final settlement under which Iran agrees to undergo sufficiently thorough monitoring to convince the international community it does not have a military programme.

Officials from the six-nation negotiating group are considering how to prevent a collapse of the diplomatic process in Moscow on 18 June, which would increase the likelihood of an Israeli strike. So far western officials have ruled out the offer of a relaxation in sanctions in return for the end of 20% enrichment.

"The Iranians have given nothing, no concessions since 2006. Why reward that with further concessions from our side?" a senior European diplomat asked.

Further US banking sanctions are due to take effect this month, followed by an EU oil embargo on 1 JulyUS secretary of state Hillary Clinton played down Khamenei's comments, saying she did not think they had any bearing on the next round of talks between Iran and the six nations, scheduled to resume on June 18-19 in Moscow."I don't draw any conclusions from that statement with respect to the potential success of the talks in Moscow," Clinton told reporters in Stockholm.