The United States does not believe Iran is capable of enriching uranium to the degree it says it is enriching, White House spokesman Robert Gibbs said today.
"Iran has made a series of statements that are ... based on politics not on physics," Mr Gibbs told reporters.
Earlier, Iranian president Mahmoud Ahmadinejad told a rally marking the 1979 Islamic Revolution that Iran was now able to enrich uranium to more than 80 percent purity, coming close to levels experts say would be needed for a nuclear bomb.
Mr Gibbs also said Google and other Internet service providers had been "unplugged" in Iran.
US State Department spokesman P.J. Crowley said Iran appeared to have tried to impose a "near total" blockade on the flow of information within the country.
He said he had seen reports that Iran's telephone network, text messaging, satellite television and the Internet had all been jammed.
Microsoft has not experienced any disruptions to its Hotmail e-mail service in Iran, a source familiar with the company said.
Mr Ahmadinejad told a huge flag-waving crowd on the anniversary of the 1979 Islamic Revolution that Iran does not want to produce a nuclear bomb but if it ever did, it would do so publicly, a response to Western concerns that Iran is seeking to develop a nuclear weapons capability in secret.
"When we say that we don't build nuclear bombs, it means that we won't do that because we don't believe in having it," he said. "The Iranian nation is brave enough that if one day we wanted to build nuclear bombs we would announce it publicly without being afraid of you."
"Right now in Natanz (enrichment complex) we have the capability to enrich to more than 20 per cent and (also) to more than 80 per cent, but because we don't need to, we won't do so.
Mr Ahmadinejad also said Iran had produced its first batch of 20 per cent-enriched atomic fuel, two days after it announced the start of the project.
Iran denies Western accusations that its nuclear energy programme has military goals, saying it only seeks to generate electricity so that it can export more of its oil and gas.
Iran opted to escalate enrichment after the collapse of efforts to iron out a fuel swap deal with the West, under which it would have sent much of its low-enriched uranium abroad in return for 20 per cent pure fuel rods for a medical reactor.
Tehran says its shift into 20 per cent enrichment is solely to replenish the medical isotope-producing reactor's fuel stock, which is due to run out of such fuel later this year.
But Iran lacks the technical means to convert 20 per cent fuel into the special fuel assemblies for the reactor, raising scepticism about its motivations for higher-scale enrichment.
The main technical challenge in enriching uranium is to reach a level of 3.5 per cent -- which Iran has already done. After that scaling up to 20, or 80 per cent, is relatively easy.
However, Iran would then still have to master technology to convert high-enriched uranium into a deliverable nuclear weapon. US National Intelligence chief Dennis Blair said last year Iran would not be capable of weaponising enrichment before 2013.
Mr Ahmadinejad said Iran would in the near future treble output of 20 per cent fuel. "Why do they (West) think by producing 20 per cent fuel a major event has happened? Right now at Natanz we have the capability to enrich uranium to much higher levels."
Iranian officials say Tehran remains prepared to exchange fuel, but under conditions which the West has rejected.
For world powers and the UN nuclear watchdog, the swap's attraction lies in preventing Iran from retaining enough of the material for a nuclear weapon, if it were refined to 90 per cent.
Iran has insisted on simultaneous exchanges of small amounts of low-enriched uranium on its own soil, which would allow it to keep enough for use in a weapon, if it so decided.
"Come and give us fuel without preconditions. We are ready to buy fuel from an country that provides us with it, we are even ready to buy fuel from America," Mr Ahmadinejad said.
Reuters