Barack Obama signalled a new era in relations with Iran yesterday but conceded that he would not be able to close Guantánamo Bay prison camp early in his presidency.

In his most wide-ranging television interview since his election, Mr Obama made clear the scale of the challenges confronting him at home and abroad. He vowed to act immediately on the US economy and the Middle East crisis, promised to engage with Iran, confront terrorism and work out a plan to close Guantánamo. Those issues are only part of a hugely ambitious agenda that some analysts believe will prove unrealistic.

Mr Obama, who will be sworn in as President next Tuesday, repeated his pledge to close Guantánamo Bay, which still holds 248 detainees, but acknowledged that it would not happen in his first 100 days in office. “It is more difficult than I think a lot of people realise,” he said. “Part of the challenge is that you have a bunch of folks that have been detained, many of whom may be very dangerous, who have not been put on trial,” Mr Obama told ABC's This Week. “And some of the evidence against them may be tainted even though it's true” - a reference to claims that some inmates were subjected to torture and harsh interrogations.

Robert Gates, President Bush's Defence Secretary, who has been asked by Mr Obama to stay in his post, ordered closure plans to be drawn up last month. Options being looked at combine a special trial system on US soil for the most dangerous inmates, while transferring other detainees overseas.

The Times revealed last month that Britain was preparing to reverse course, having rejected overtures by the Bush Administration to take detainees, and was considering looking at the issue of receiving Guantánamo inmates on a “case by case basis”.

Despite a growing willingness within the EU to help Mr Obama to close Guantánamo, he is still confronted with enormous popular resistance abroad to the idea of repatriating as many as 200 inmates overseas. He is also faced with the reality that even if foreign governments do co-operate, they will move slowly and might only be willing to take a handful of inmates.

Last week William Perry, a former Clinton-era Defence Secretary, said that Mr Obama was likely to face a “serious crisis” with Iran over its nuclear ambitions in his first year in office.

Mr Obama did not disagree with that analysis yesterday, saying: “I think Iran is going to be one of our biggest challenges.” He said that Tehran was exporting terror through Hamas and Hezbollah, and “they are pursuing a nuclear weapon that could potentially trigger a nuclear arms race in the Middle East”.

However, Mr Obama said that the US had to take a “new approach” with Iran, with a “new emphasis on being willing to talk”. He added: “We anticipate that we're going to have to move swiftly in that area.”

Mr Obama has faced growing criticism in recent days for his refusal to comment on the Israeli offensive in Gaza. He has defended his silence by saying that US foreign policy can only be directed by one President at a time. He vowed yesterday to engage with the Middle East “on day one”, but indicated that his policy towards Israel would not substantially change from the Bush Administration's.

He stood by his comment made during a visit in July to the southern Israeli town of Sderot - a frequent target of Hamas rockets - when he said: “If somebody was sending rockets into my house where my two daughters sleep at night, I'm going to do everything in my power to stop that. I would expect Israelis to do the same thing.”

Asked if he would repeat that remark now, he said: “I think that's a basic principle of any country that they've got to protect their citizens.” Asked if he would make a clean break with Mr Bush over Middle East policy, Mr Obama said: “I think that, if you look not just at the Bush Administration but what happened under the Clinton Administration, you are seeing the general outlines of an approach.”

Mr Obama pledged, however, to engage in the Middle East immediately, and not to wait until the end of his term - a reference to Bill Clinton's failed effort to forge an Israeli-Palestinian peace deal in the final days of his presidency.

The challenges

— Middle East peace process

— US economy

— Engaging Iran

— Closing Guantánamo

— Fighting terrorism

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