Supreme leader Ayatollah Ali Khamenei slammed US President Barack Obama on Sunday for threatening a "nuclear attack" even as Defence Secretary Robert Gates said he did not believe Iran had an atomic bomb.

Foreign ministry spokesman Ramin Mehmanparast said Iran will officially complain to the United Nations regarding Obama's "threats" after 225 members of parliament asked the government to take up the issue.

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

"He (Obama) has implicitly threatened Iranians with nuclear weapons," 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," said Iran's spiritual guide.

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

In a policy shift, Washington said on Tuesday it would use atomic weapons only in "extreme circumstances" and would not attack non-nuclear states -- but singled out "outliers" Iran and North Korea as exceptions.

Defence Secretary Robert Gates 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," Gates said.

After a year of attempting diplomatic initiatives, Obama has in recent weeks ratcheted up pressure for fresh UN sanctions against Iran over its nuclear programme, which Washington suspects is masking a weapons drive.

Khamenei dismissed Washington's policy as passing "tornados."

"After 30 years, the Iranian nation has shown that it is more resilient and strong and has the ability to stand against any kind of threat," the cleric said.

"Our armed forces must also be alert towards such threats and take their training seriously."

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

On Friday, Iran unveiled a third-generation centrifuge it said can enrich uranium six times faster than the IR-1 system currently installed at its plant in the central city of Natanz.

The Natanz facility has a capacity of 60,000 centrifuges, and Iran has been steadily enriching uranium there for years in defiance of three sets of UN sanctions and the threat of a fourth.

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 defence secretary said on Sunday that Washington did not believe that Tehran yet had a nuclear weapons capability.

"It's going slower... than they anticipated. But they are moving in that direction," he told NBC's "Meet the Press."

"I'd just say, and it's our judgment here, they are not nuclear capable," Gates added. "Not yet."

Secretary of State Hillary Clinton, who appeared along with Gates on television political talk shows, 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.'"

On Thursday, Washington secured Beijing's agreement to further talks among the major powers on new UN sanctions against Tehran.

A key 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.