The leaders of Iran and the United States competed on Tuesday with sharply worded messages for the start of the Iranian new year that highlighted global tensions over Tehran's controversial nuclear programme.

Iran's supreme leader, Ayatollah Ali Khamenei, warned in a speech broadcast live on state television: "We have said that we do not have atomic weapons and we will not build any.

"But if there is any attack by the enemies, whether it be United States or the Zionist regime (Israel), we will attack them at the same level as they attack us."

He claimed the real reason for the hostility of Western governments was that they wanted to wield control over Iran's vast oil and gas reserves.

"We will not allow them to do so, and they will remain our enemies," he said.

"Those who think, if we yield on the nuclear issue, then US hostility towards us will decrease, they are wrong. Their case (the US case) against us is not the nuclear programme nor is it human rights. It is that the Islamic Republic of Iran is standing against them," he said.

Simultaneously, US President Barack Obama issued his own video message via YouTube, acknowledging the "continued tension between our two countries" -- and vowing to break Tehran's sophisticated internet censorship so Iranians can "access the information that they want."

On top of US sanctions already imposed, Obama said Washington is encouraging "American businesses to provide software and services into Iran that will make it easier for the Iranian people to use the Internet."

The high-level jostling for Iranians' attention came on Nowruz, the first day of the Iranian new year that occurs with the transition to spring and which traditionally represents a period of renewal.

Iran, in a symbolic gesture underlining its readiness to counter any military threat, fired a shell from its flagship destroyer to mark the precise beginning of the new year.

Khamenei, in the first of two speeches to the Iranian people, declared the next 12 months to be a year of "domestic production and the support of Iranian investment and labour."

He cast that as a measure of defiance against US and EU sanctions that are weighing on Iran's economy, which has long been struggling with high inflation and unemployment.

If Iran boosts its economy, Khamenei said, "the enemy (the West) will despair and its efforts to plot and conspire against us will end."

He said Iran had, in the previous 12 months, forged ahead in nuclear research and many other fields "despite all the animosity, all the propaganda, all the hostile attacks and ill-wishes" of the West.

Israel, he asserted, was isolated by the uprisings in the Arab world that have seen pro-US regimes toppled and replaced by more Islamist administrations.

In his second, longer speech from the northeastern city of Mashhad, Khamenei said Iran had a divine duty to strike back at any aggressors.

He also said that Iran's oil wealth -- which is being targeted by US and EU sanctions -- would eventually allow the Islamic republic to prevail over its enemies.

"When the day comes that they cannot obtain any more oil and gas, that will be the day they will have to make concessions, and it will be catastrophic for them," he said.

Obama's pre-recorded message -- carried in English, Farsi and Arabic on YouTube -- ignored Khamenei and spoke directly to the Iranian people.

He said their government could end its international isolation if it took a "responsible path" on its nuclear programme.

He also accused Iran's leaders of imposing an "electronic curtain" of censorship through its aggressive filtering of the Internet, jamming of foreign television and radio broadcasts, and monitoring of computers and cellphones.

He vowed to pierce through that "curtain" with new US software and through social media such as Skype, Yahoo Messenger, Google Talk and using browsers like Firefox and Internet Explorer.

Obama also highlighted a "Virtual US Embassy" online that was blocked in Iran hours after it was launched in early December. A similar British initiative launched last week was also blocked.

Iran and the United States have no diplomatic relations. Washington has repeatedly urged Iran's clerical regime to improve its record on human rights, including treatment of political critics and religious minorities.

Iran last month agreed to revive talks between it and the P5+1 group of powers -- the five UN Security Council permanent members plus Germany -- but as yet no date or venue for the negotiations has been announced.

The previous round of Iran-P5+1 talks collapsed in Istanbul in January last year.

'Window' closing to kick off talks with Iran: Ashton
Brussels (AFP) March 20, 2012 - EU foreign policy chief Catherine Ashton, who heads global nuclear talks with Iran, Tuesday said it was urgent to return to negotations as "the window of opportunity is closing."

Ashton, who represents six world powers seeking to convince Iran to freeze its disputed nuclear programme, insisted in a hearing with the EU parliament's foreign affairs committee that she would look for a solution when talks with Tehran finally resume.

"I am looking for a solution. I don't believe it will be done in one discussion. I do believe it can be done," she added.

On behalf of Britain, China, France, Germany, Russia and the United States, Ashton has offered to resume stalled talks with Iran but a time and venue still need to be agreed.

Western powers suspect Iran is seeking to build a nuclear bomb, a charge denied by Tehran, which says its atomic programme is for peaceful purposes.

After several failed rounds, the last in Istanbul just over a year ago, the powers agreed to resume the talks after receiving a written commitment by Iran to address the nuclear issue at the negotiations.

In Istanbul, Iran refused to address questions on its nuclear programme, demanding what diplomatic sources dubbed "pre-conditions", such as the lifting of sanctions.

But in a February 14 letter to Ashton, Iran's chief nuclear negotiator Saeed Jalili said Tehran was ready to resume the deadlocked talks as long as its right to peaceful atomic energy was respected.

"We're ready to sit down with them" on the basis of Jalili's letter, Ashton said.

"But you will also appreciate that we are talking with a purpose and it will be very, very important that it becomes clear quickly that there is a real purpose to these talks," she said.

"That for me is going to be the basis upon which we will start; with respect to Iran, but with a clarity that we want to see progress within a good reasonable time."

Jalili's letter, a long-belated response to one from Ashton in October, came as world powers moved to adopt unprecedented economic sanctions against Tehran, including an EU oil ban due to come into force on July 1.

Sanctions were ramped up in the last months after the UN's International Atomic Energy Agency said in a report it had evidence the Islamic republic appeared to be conducting research on atomic warheads.

"It's very, very important that we have sanctions that are designed to persuade Iran to come to the table," Ashton said, but added: "The purpose is not to hurt the population of Iran."