Speaking in Brussels on Tuesday, Mark Dubowitz warned that Iran was in the process of installing "thousands of new, more efficient centrifuges".
He said, "By June 2014, they'll have so many centrifuges spinning they'll be able to achieve undetectable nuclear breakout. The challenge is to fashion a deal with Iran that puts strict limits on its centrifuges."
Dubowitz is director of the foundation for defence of democracies think tank, and an expert on Iran.
The west has imposed several rounds of punishing sanctions on Iran, which Iranian president Mahmoud Ahmadinejad says has crippled its economy.
Western powers are hoping to force Iran to scale back its uranium enrichment programme, suspecting it might aim to produce nuclear weapons.
Iran has refused, insisting its programme is for peaceful purposes, and says it has a right to enrich under the nuclear non-proliferation treaty.
Dubowitz, who was briefing Brussels-based journalists on the current situation in Iran, said, "The Iranians have created the perception that their 20 per cent enriched uranium stockpile is so valuable to them that they won't give it up without extracting a major concession from the international community, such as lifting all of the sanctions.
"But the stockpile isn't that important because they're building the capacity to enrich uranium to any level in a short amount of time.
"They'll soon have so many centrifuges that they can produce enough weapons-grade uranium in a week or two, between visits by UN inspectors."
Dubowitz said Europe can play a key role in ensuring that sanctions against Iran will prove effective, adding, "Iran is now closer and closer to that breakout capacity where it can produce enough weapons grade uranium without our being able to stop them.
"They may also be capable of a nuclear 'sneak out' if they have a secret enrichment facility. And they are working on a plutonium path to a nuclear weapon using their Arak heavy water reactor."
He added, "If sanctions don't intensify and persuade the Iranian regime to reach a peaceful resolution of this Iranian nuclear crisis, the Obama administration will launch military strikes to stop Iran before it reaches an undetectable nuclear weapon."
He said, "Sanctions have succeeded in getting Iran to the negotiation table. The regime is negotiating because the sanctions are causing serious economic stress. That is why Iran is demanding major sanctions relief in exchange for nuclear concessions.
"But sanctions have not changed the nuclear calculus of Iran's 'supreme leader'. The sanctions need to be so punishing that they will bring the Iranian regime to the brink of economic collapse."
Faced with a choice between "economic collapse", he said Iran's leader will "choose his political survival over compromise".
He added, "It may be that the regime is so committed to a nuke that even an imminent economic meltdown won't break the nuclear will of Iran's revolutionary guards.
"But for the west to even test that thesis, it needs to bring Iran to the brink."