Iran is engaged in a standoff with the West over its uranium enrichment programme, which it says is for the peaceful generation of electricity but which Western countries say is a cover to build an atomic weapon.

Presidential pledges

Gibbs also denied a report in the British newspaper Guardian that said Obama and Hillary Clinton, the US secretary of state, were drafting a letter to Iran's leadership to push ahead with the administration's pledge of diplomatic engagement.

In depth
"Neither the president nor the secretary of state has requested nor seen any such letter. That closes the book a little bit on that," he said.

Hillary Mann Leverett, a former US state department official, told Al Jazeera that any letter from the Obama administration was likely to be addressed to the Iranian people, but that could look like a deliberate attempt to undermine Iran's government, "which is something that clearly doesn't work".

Obama said during his presidential election campaign that he would consider holding talks with countries that have poor relations with the US, such Iran and Cuba.

During the campaign, Obama said he would offer Iran various incentives, including helping it to join the World Trade Organisation, if it stopped its nuclear activities.