TEHERAN, Iran - Iran's Supreme Leader Ayatollah Ali Khamenei said on Friday that Iran would retaliate over Western-backed oil sanctions and any threat of attack, after US Defense Secretary Leon Panetta said he feared a possible Israeli strike as early as April.

Khamenei's defiant speech to mark the anniversary of the 1979 Islamic Revolution was the first direct response to tighter sanctions imposed by the West in recent weeks to force Teheran to abandon a nuclear program it says has purely peaceful ends.

"Threatening Iran and attacking Iran will harm America ... Sanctions will not have any impact on our determination to continue our nuclear course ... In response to threats of oil embargo and war, we have our own threats to impose at the right time," Khamenei told worshippers in a speech broadcast live on state television.

US media reports said US Defense Secretary Panetta believed there was a growing possibility Israel would attack Iran as early as April to stop it from building a nuclear bomb.

The Washington Post first reported on Thursday that Panetta was concerned about an increased likelihood Israel would launch an attack over the next few months. CNN said it confirmed the report, citing a senior Obama administration official, who declined to be identified.

"Panetta believes there is a strong likelihood that Israel will strike Iran in April, May or June - before Iran enters what Israelis described as a 'zone of immunity' to commence building a nuclear bomb," columnist David Ignatius wrote.

"Very soon, the Israelis fear, the Iranians will have stored enough enriched uranium in deep underground facilities to make a weapon - and only the United States could then stop them militarily," Ignatius wrote.

Panetta and the Pentagon both declined comment on the Post report.

Khamenei said any US military strike against Iran would backfire and that the "painful and crippling" Western sanctions would only increase the resilience of Iran.

"Americans say all options are on the table even the option of military strike (against Iran) ... Any military strike is 10 times more harmful for America. Such threats show that they have no sufficient discourse against Iran's logic and discourse."

Reuters