Ahmadinejad will deliver his address at the United Nations some time after Bush, and like in 2006, when he may launch anew another anti-American diatribe.

It has been two years since Ahmadinejad made his presence felt on the world stage with his first appearance at the United Nations, and already icy relations between Washington and Tehran have gotten worse.

The UN Security Council has imposed two resolutions against Iran for its refusal to renounce its disputed nuclear program. And Ahmadinejad will make his speech as Bush pushes the Security Council for tougher sanctions against Tehran.

Ahmadinejad defends his country's nuclear program as solely designed to generate electricity. But Bush warns that Iran is seeking to build an atomic arsenal and, with a hardliner like Ahmadinejad in power who has called for the destruction of Israel and tried to cast doubt on the Holocaust, could use the bomb against Washington's close ally.

On the eve of his departure for the United States -- where his visit has sparked a chorus of protests -- Ahmadinejad struck a defiant tone, saying in a speech that warnings of military action and more UN sanctions would have no effect on Tehran's nuclear drive.

"Those who think that with outmoded instruments like psychological warfare and economic sanctions they can stop Iran's march towards progress are making a grave mistake," Ahmadinejad said.

But in an interview with a US television network, Ahmadinejad played down rising tensions and said the two countries were not headed for war.

"You have to appreciate we don't need a nuclear bomb. We don't need that. What need do we have for a bomb?" the Iranian leader said, according to a transcript released by CBS on Sunday.

"It's wrong to think that Iran and the US are walking towards war. Who says so? Why should we go to war? There is no war in the offing."

Given the escalating rhetoric, speculation has mounted that Bush may choose to take military action against Iran before the end of his presidential term in January 2009.

French Foreign Minister Bernard Kouchner this month spoke openly of a possible war if diplomacy with Iran failed.

According to Bush, who once named Iran as part of an "axis of evil," the Tehran regime is allegedly arming militant Shiites fighting US soldiers in Iraq, helping the Taliban in Afghanistan, supporting Hezbollah in its bid to undermine the Lebanese government and backing anti-Israeli Palestinian groups labeled by the US as terrorists.

The US military made fresh accusations against Iran on Sunday charging Tehran was supplying insurgents with sophisticated ground-to-air missiles to target American troops.

The Iranian president, in another move sure to irritate Bush, has asked to lay a wreath at the site of the World Trade Center, destroyed in the September 2001 attacks.

Bush said he could "understand" why city officials denied Ahmadinejad's request, as the Iranian president leads a country that is a "state sponsor of terror."

Bush has refused to rule out the use of force over Iran's nuclear program but for the moment has called for pursuing a diplomatic solution.

"I am hopeful that we can convince the Iranian regime to give up any ambitions it has in developing a weapons program, and do so peacefully," Bush said on Thursday.

The US president has so far rebuffed calls to renew dialogue with Tehran after a 27-year gap. US-Iran relations collapsed in the wake of the 1979 seizure of the American embassy in Tehran.

Ahmadinejad meanwhile repeated his offer, first made in 2006, to have a debate with Bush at the UN. The White House has ruled out any such event.

Top US diplomat Condoleezza Rice and her Iranian counterpart Manouchehr Mottaki did not speak to each other on Saturday during a UN conference on Iraq, a US official said.

Ahmadinejad said his visit to the UN General Assembly and before an audience at Columbia University in New York would offer a chance "to present the positions of the Iranian people as they (the Americans) are very keen to hear them."