However the head of Iran's Revolutionary Guards insists the world knows Tehran is not pursuing nuclear arms.

The threats to ramp up enrichment made by Tehran's nuclear agency spokesman would go far beyond the small steps Iran has taken in the past week to nudge stocks of fissile material just beyond limits in the pact, which Washington abandoned last year.

They would reverse the major achievements of an agreement intended to block Iran from making a nuclear weapon, and raise serious questions about whether the accord is still viable.

Head of the Revolutionary Guards Major General Hossein Salami denied Iran was pursuing a nuclear weapon.

"Why do they globally sanction us about the nuclear issue when the world knows that we are not pursuing a weapon? In reality they are sanctioning us because of knowledge," Salami was quoted as saying by the semi-official Tasnim news agency on Monday.

"Nuclear weapons have no place in Islam. Islam never approves of weapons of mass destruction," he added.

Despite the Iranian threat to boost enrichment, the US said its renewed sanctions against Tehran had been effective.

"President Trump's maximum pressure campaign against Iran is working," White House national security adviser John Bolton told a pro-Israel group in Washington.

"We're just getting started ... The president's goal is to get a new deal that would be negotiated in the best interests of the United States."

US Vice President Mike Pence, who spoke to the same group, added a warning.

"Iran should not confuse American restraint with a lack of American resolve," he said.

Spokesman for Iran's Atomic Energy Organisation Behrouz Kamalvandi confirmed Tehran had enriched uranium beyond the 2015 deal's limit of 3.67 per cent purity, passing 4.5 per cent, according to news agency ISNA.

UN nuclear watchdog the IAEA confirmed it had verified Iran's enrichment was beyond 3.67 per cent.

Iran has said it will take another, third step away from the deal within 60 days.

Kamalvandi said options included enriching uranium to 20 per cent purity or beyond, and restarting IR-2 M centrifuges dismantled under the deal.

French President Emmanuel Macron was sending his top diplomatic advisor to Iran on Tuesday and Wednesday to try to help defuse tensions, a presidential official said.

The White House said Trump had spoken to Macron on Monday to discuss efforts to ensure Iran does not obtain a nuclear weapon.

Enriching uranium up to 20 per cent purity would be a dramatic move, since that was the level Iran achieved before the deal.

However back then it had a far larger stockpile than it is likely to be able to rebuild in the short term.

Twenty per cent purity is considered an important intermediate stage on the path to obtaining the 90 per cent pure fissile uranium needed for a bomb.

Australian Associated Press