Iran could develop a nuclear bomb in about a year and create the means for delivery in a further two to three years, the US defense chief said Sunday, reiterating President Barack Obama's determination to halt the effort.

"The United States -- and the president's made this clear -- does not want Iran to develop a nuclear weapon," Defense Secretary Leon Panetta told the CBS program "60 Minutes."

"That's a red line for us. And it's a red line obviously for the Israelis so we share a common goal here."

Panetta maintained that US officials "will take whatever steps are necessary to stop it" if Washington receives intelligence that Iran is proceeding with developing a nuclear weapon.

Asked if that meant military action, he said: "There are no options that are off the table."

Panetta told the interviewer that "the consensus is that, if they (Iran) decided to do it, it would probably take them about a year to be able to produce a bomb and then possibly another one to two years in order to put it on a deliverable vehicle of some sort in order to deliver that weapon."

In a report issued in November, the International Atomic Energy Agency said intelligence from more than 10 countries and its own sources "indicates that Iran has carried out activities relevant to the development of a nuclear device."

It detailed 12 suspicious areas such as testing explosives in a steel container at a military base and studies on Shahab-3 ballistic missile warheads that the IAEA said were "highly relevant to a nuclear weapon program."

Iran rejected the dossier as based on forgeries.

The Islamic Republic has come under unprecedented international pressure since the publication of the report, with Washington and the European Union targeting its oil sector and central bank.

In his State of the Union message Tuesday, Obama said a peaceful outcome was still possible with Iran over its nuclear ambitions, but he declined to rule out the military option.

"The regime is more isolated than ever before; its leaders are faced with crippling sanctions, and as long as they shirk their responsibilities, this pressure will not relent," Obama said.

"Let there be no doubt:America is determined to prevent Iran from getting a nuclear weapon, and I will take no options off the table to achieve that goal," the president declared, triggering a standing ovation.

UN chief nuclear inspector arrives in Iran
Tehran (AFP) Jan 29, 2012 - The UN's chief nuclear inspector arrived in Iran on Sunday on a mission to clear up "outstanding substantive issues" on Tehran's atomic programme, and called for dialogue with the Islamic state.

Before departing from Vienna airport, International Atomic Energy Agency (IAEA) chief inspector Herman Nackaerts told reporters that talks were long overdue.

"We are trying to resolve all the outstanding issues with Iran," he said.

"In particular we hope that Iran will engage with us on the possible military dimensions of Iran's nuclear programme. We are looking forward to the start of a dialogue, a dialogue that is overdue since very long."

Nackaerts is leading a six-person IAEA team due to meet Iranian officials from later Sunday until Tuesday. The delegation touched down in Tehran early on Sunday morning, the official news agency IRNA reported.

The team also includes IAEA number two Rafael Grossi, an Argentine, and the watchdog's senior legal official Peri Lynne Johnson, a US citizen, according to diplomats.

Nackaerts, who is Belgian, declined to comment on who he would meet during the trip, which is aimed at clearing up what the IAEA called "outstanding substantive issues" on Tehran's nuclear programme.

Expectations are low, with the delegation not expected to be given access to any sites mentioned in a damning IAEA report in November that raised suspicions Iran had done work developing nuclear weapons.

IRNA quoted Iranian Foreign Minister Ali Akbar Salehi, in Addis Ababa for an African Union meeting, as saying he was "optimistic" about the delegation's visit.

"We have always had a broad and close cooperation with the agency and we have always maintained transparency as one of our principles working with the agency," he said.

The agency added that the team would probably visit the Fordo enrichment site south of the capital Tehran.

Earlier this month, the IAEA said Iran had begun enriching uranium to 20 percent purity deep inside a mountain bunker at Fordo, taking it significantly closer to the 90 percent mark needed for a nuclear bomb.

With Iran repeatedly denying it wants nuclear weapons and dismissing the IAEA report as baseless, the watchdog's chief Yukiya Amano on Friday urged the Islamic republic to show "substantial cooperation" during the visit.

The report, which has led to a substantial increase in pressure on Iran from the United States, the European Union and others, detailed a string of areas in which it said Iranian activities were highly suspicious.

"We have information that indicates that Iran has engaged in activities relevant to the development of a nuclear explosive device," Amano said on Friday at the World Economic Forum in Davos, Switzerland.

"We are requesting that Iran clarifies the situation. We proposed to make a mission and they agreed to accept the mission. The preparations have gone well but we need to see what actually happens when the mission arrives."

Iran's President Mahmoud Ahmadinejad has insisted that Tehran is not dodging negotiations and was ready to sit down with world powers Britain, China, France, Russia, the United States, and Germany for talks.

The six powers are waiting for Tehran to reply to an October letter sent by EU foreign policy chief Catherine Ashton that stresses that discussions should focus on the "key question" of the Iranian nuclear issue.

IRNA said Sunday that Iran's top nuclear negotiator Saeed Jalili will write a letter to Ashton which "will give Iran's views about the time and venue of the (next) negotiations".

The letter "which may be sent soon, in the coming days", it quoted the foreign minister as saying, adding the next round of talks were likely to be a "success since both sides are interested to find a solution for Iran's nuclear issue."

Previous talks held a year ago in Istanbul ended without progress.

UN chief Ban Ki-moon, also in Davos, said the onus was on Iran to prove its good intentions.

"There is no other alternative to addressing this crisis than peaceful resolution through dialogue," Ban told reporters.

What makes the international community all the more nervous is Tehran's continued defiance of UN Security Council resolutions calling on it to stop enriching uranium until the IAEA is satisfied its programme is peaceful.