SDEROT, Israel/WASHINGTON/ TEHRAN: US presidential hopeful Barack Obama said on Wednesday that a nuclear Iran would pose a “grave threat” and that the world must prevent it from obtaining the atomic bomb.

“A nuclear Iran would pose a grave threat and the world must prevent Iran from obtaining a nuclear weapon,” Obama said.

“A nuclear Iran would be a game-changing situation, not just in the Middle East but around the world,” Obama said on the latest leg of talks in Israel and the Palestinian territories.

Obama spoke of a series of “big sticks and big carrots” regarding the the Iranian regime and repeated his openness to meeting with its representatives, if the conditions were appropriate.

“I would at my time and choosing be willing to meet with any leader if I thought it would promote the national security interest of the United States of America,” he said.

Meanwhile, Iranian President Mahmoud Ahmadinejad on Wednesday vowed that Iran would not yield in the crisis over its nuclear drive but the White House warned again that it risks more sanctions if it fails to freeze sensitive nuclear work.

“The Iranian people are steadfast and will not step back an inch against the oppressive powers,” Ahmadinejad told a rally in the south-western province of Kohgelouyeh-Boyerahmad.

His defiant comments come after world powers warned Iran has only a fortnight to respond to their latest offer seeking to end a five-year crisis that has raised fears of regional conflict and sent oil prices spiralling.

The White House said it was still hopeful Tehran would suspend its controversial nuclear activities, otherwise more international sanctions would follow.

“We hope the Iranians will provide a positive answer,” national security spokesman Gordon Johndroe said.

But “if they do not, the international community is united that more sanctions are coming.”

World powers have offered to start pre-negotiations during which Tehran would add no more uranium-enriching centrifuges and in return face no further sanctions—the so-called “freeze-for-freeze” approach.

Iran is already under three sets of UN Security Council sanctions over its refusal to halt sensitive uranium enrichment work, which the West fears could be aimed at making nuclear weapons.

The United States took the unprecedented step of sending a top diplomat to meet Iran’s chief negotiator Saeed Jalili at Saturday’s international talks in Geneva which ended in a stalemate.

But Washington, which has led the efforts to thwart Iran’s nuclear drive, has also warned Tehran of “punitive measures” if it spurns the international offer and presses on with enrichment work.

Ahmadinejad welcomed the US presence in talks as a “positive step” and said that the US diplomat William Burns had talked “politely in the meeting and respected the Iranian nation.” But the president vowed that further sanctions would not force Iran to back down in the standoff.

“The Iranian nation does not value your threats. You are mistaken if you think you can force this nation to back down with sanctions, threats and pressure,” he said.

Iran denies allegations of seeking nuclear weapons, insisting that its programme is designed to provide energy for its growing population when the OPEC member’s reserves of fossil fuels run out.

Permanent Security Council members—Britain, China, France, Russia and the United States—plus Germany have made Iran an offer, which includes trade incentives and help with a civilian nuclear programme in return for suspending enrichment.

Ahmadinejad alleged that world powers “do not mind centrifuges and enrichment... but that they want to show the world they can force the Iranians to retreat with a psychological operation and deception.”

The New York Times on Tuesday released what it said was a two-page informal document that outlined Tehran’s approach to talks in Geneva and was distributed by Iranian negotiators.

The paper called for seven more rounds of talks, stressed the need for an end to sanctions, and made no mention of an incentives package.

Iranian officials have repeatedly said they have no intention of freezing enrichment and that as a signatory to the nuclear Non-Proliferation Treaty, Iran has the right to make its own nuclear fuel.