Iran's supreme leader says his country will assist any nation or group that "confronts" Israel, describing it as a "cancer."
The remarks by Ayatollah Ali Khamenei came during a Friday prayer ceremony in Tehran broadcast on state television.
Khamenei, who has final say on all state matters, says that Iran has helped the Lebanese militant group Hezbollah and the Palestinian Hamas in their fights against Israel, and will aid others who do so.
He describes Israel as a "cancer" and says, "It will be cut."
Israel and Iran consider each other arch-enemies.
Khamenei said on Friday that Iran would retaliate over Western-backed oil sanctions and any threat of attack, after U.S. Defense Secretary Leon Panetta was cited as saying 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 Tehran 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.
"I have no fear of saying that we will back and help any nation or group that wants to confront and fight against the Zionist regime [Israel]."
U.S. media reports said 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.
"PAINFUL AND CRIPPLING"
Khamenei said any U.S. 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 a 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."
"Such threats show that America has no way but using force and bloodshed to achieve its goals, which further harms America's rulers, international and domestic credibility," he added.
Khamenei said the aim of the sanctions was to punish "the Islamic Republic because of Islam."
"Such sanctions will benefit us. They will make us more self-reliant. ... We would not achieve military progress if sanctions were not imposed on Iran's military sector ... More imposed pressures mean more self-reliance for Iran."
"Sanctions are beneficial also because it makes us more determined not to change our nuclear course ... Iran will not change its nuclear course because of sanctions...," he added.
Israel, widely believed to possess the Middle East's only nuclear arsenal, views Iran's uranium enrichment projects as a major threat and has not ruled out the use of military force to prevent Tehran from acquiring nuclear weapons.
The Post said the postponement of a joint U.S.-Israeli military exercise that had been scheduled for this spring may have signaled the prospect of an Israeli attack soon.
Iran has said repeatedly it could close the vital Strait of Hormuz oil export route if sanctions succeed in preventing it from exporting crude, a move Washington said it would not tolerate.
Israel's military intelligence chief, Maj. Gen. Aviv Kochavi, said on Thursday he estimated that Iran could make four atomic bombs by further enriching uranium it had already stockpiled, and could produce its first bomb within a year of deciding to build one.
Citing figures similar to those from the IAEA, the U.N. nuclear agency, Kochavi told Israel's annual Herzliya Conference on strategic affairs: "Iran has accumulated more than 4 tons of uranium enriched to a level of 3.5 percent and nearly 100 kilos at an enrichment level of 20 percent.
"This amount of material is already enough for four atomic bombs."
Israeli Defense Minister Ehud Barak said separately that "if sanctions don't achieve the desired goal of stopping [Iran's] military nuclear program, there will be a need to consider taking action."
A top Chinese newspaper stepped up Beijing's opposition to a Western push for tighter sanctions against Iran, warning on Friday that tensions over Tehran's nuclear program are hurting energy markets and could stifle the global economic recovery.
China's criticism appeared in the People's Daily, the mouthpiece of the ruling Communist Party. It comes a day after German Chancellor Angela Merkel urged Beijing to use its influence to persuade Iran to give up its nuclear program.
"The global economy is in the midst of a difficult economic recovery and reducing the shocks of uncertainties is the common responsibility of countries all over the world," the People's Daily commentary said.
"In the near term, the sudden spike in tensions between the United States and Iran is now posing the greatest uncertainty. This factor is disrupting global energy markets and has cast a shadow over the global economic recovery."
China, the world's second-largest crude consumer and the biggest buyer of Iranian oil, has long opposed unilateral sanctions that target Iran's energy sector and has tried to reduce tensions that could threaten its oil supply.
Escalating tensions between Iran and the West have pushed up Brent crude prices by about 9 percent since mid-December.
On Thursday, at a joint media briefing after what Germany's Merkel described as "long discussions" about Iran, Chinese Premier Wen Jiabao appeared to reject the pressure to do more.
He said Beijing objected to Western nations politicizing the "normal commercial relationship" it has with the Islamic Republic, echoing language that China has used before.
Merkel, who is in China on a three-day visit, said she hoped the U.N. Security Council could pass a unanimous resolution on the Iran issue.
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