Earlier, speaking to al-Alam, Iran's Arabic-language state television station, Ali Akbar Salehi, the head of the country's atomic energy organisation, said Tehran planned to build 10 new facilities over the next year where the enrichment could be carried out.
Iran had said in November that it planned to build the enrichment plants but had not given a timescale.
PJ Crowley, a spokesman for the US state department, told Al Jazeera that Washington was "looking at how we can apply pressure on Iran, on the government itself".
"We have no interest in creating additional hardships on the Iranian people but we'll be looking at a variety of options particularly focused on the Revolutionary Guard Corps that's played an increasing role in not only Iran's security but also its economy.
"We should try to support the Iranian people … but certainly we have to look at ways in which we can apply pressure on the government and its various entities so that should Iran continue to act in contradiction to UN Security Council resolutions, that it will pay a price for that intransigence."
Recognising that "there is a legitimate humanitarian need in having additional fuel that can provide the Tehran research reactor the ability to produce medical isotopes that have a valuable role for the Iranian people", Crowley said the US had not "given up on engagement".
"We're willing to sit down with Iran as we did last fall in a good faith effort - Iran has to be willing to meet us halfway," he said.
Bernard Kouchner, the French foreign minister, said on Monday that he did not believe Iran had the ability to raise the enrichment level of its uranium and that the move by Tehran was "blackmail".
"One could call it diplomacy, but if that is what is then it is truly negative," said Kouchner.
Gates also said more pressure had to be applied on the Iranian government.
"The only path that is left to us at this point, it seems to me, is that pressure track but it will require all of the international community to work together," Gates said.
Kouchner said all the major powers apart from China were in favour of a fourth round of UN-backed sanctions.
Western powers accuse Iran of attempting to build nuclear weapons but Tehran says its nuclear programme is for purely civilian purposes.
Alireza Ronaghi, Al Jazeera's correspondent in Tehran, said: "It's a new phase in Iran's nuclear achievement, but it is not going to happen overnight.
"A very difficult design process will have to take place. They'll need to change the existing capacity that consists of 4,000 - 5,000 centrifugal machines.
|Ahmadinejad appears to have sent mixed signals in the past week [AFP]
"It will also infringe upon Iran's current capacity for enriching uranium to a level of 3.5 - 4 per cent, which is necessary for its current nuclear programme.
"Regardless, these are the things that many conservatives in Iran think will make the country's hand stronger in negotiations with the West."
Iran has expressed its readiness to exchange its low-enriched uranium for higher-grade fuel, but has demanded amendments to the UN-drafted IAEA plan, under which Iran would export its low-enriched uranium abroad for enrichment.
The UN plan was drawn up in early October in a meeting in Geneva, Switzerland, between Iran and six world powers - the UK, China, France, Russia, the US and Germany - and later refined at a meeting in Vienna.
The talks in the Austrian capital came up with a draft proposal that would take 70 per cent of Iran's low-enriched uranium to reduce its stockpile of material that could be enriched to a higher level, and possibly be used to make nuclear weapons.
That uranium would be returned about a year later as refined fuel rods, which can power reactors but cannot be readily turned into weapons-grade material.
Ahmadinejad had last week appeared to support the deal in an interview on state television, but on Sunday he blamed the West for the stalemate over the deal.