The US defence secretary said in a television interview on Sunday that Washington was making exceptions of Tehran and Pyongyang because they had defied repeated UN Security Council ultimatums over their nuclear programmes.

"Well, because they're not in compliance with the nuclear Non-Proliferation Treaty. So for them all bets are off. All the options are on table," Robert Gates said.

Hillary Clinton, the US secretary of state, for her part, argued that Washington's "patience" had helped build international support for sanctions against Iran.

Clinton told NBC that "what we have found over the last months, because of our strategic patience, and our willingness to keep on this issue, is that countries are finally saying, 'You know, I kind of get it ... they're the ones who shut the door, and now we have to do something'".

'Disgraceful statements'

The US statements on the nuclear issue were condemned on Sunday by Iran's supreme leader Ayatollah Ali Khamenei, who described Obama's language a proof that the US cannot be trusted.

Khamenei, who is also the commander-in-chief of Iran's armed forces and final decision maker on crucial policy issues, told a meeting of the military's top brass to be more "alert" about such threats.

in depth
 

Timeline: Iran's nuclear  programme

  Video: Iranian view of nuclear standoff
  Video: Changing tack on uranium
  Inside Story: Sanctioning Iran
  Interview: Iran's nuclear ambitions 
  Fears grow over nuclear sites
  Q&A: Uranium enrichment
  Blog: A new focus

"He [Obama] has implicitly threatened Iranians with nuclear weapons," Iran's state television quoted Khamenei as saying.

"These comments are very strange and the world should not ignore them because in the 21st century ... the head of a state is threatening a nuclear attack.

"The US president's statements are disgraceful. Such comments harm the US and they mean that the US government is wicked and unreliable."

Ali Akbar Salehi, the Iranian atomic chief, said on Saturday that Tehran would in the coming months begin mass-producing centrifuges capable of enriching uranium three times faster than existing systems.

Uranium enrichment lies at the heart of Western concerns over Iran's nuclear programme as the sensitive process can produce fuel for a reactor or, in highly extended form, the fissile core of an atomic bomb.

The US secured China's agreement on Thursday to further  talks among the major powers on new UN sanctions against Tehran.

An important trade partner of Iran, China had previously been the  leading obstacle to adoption of a fresh sanctions package by the  Security Council among its five veto-wielding permanent members.