Iran warned world powers they won't be able to negotiate a better deal than the landmark 2015 nuclear agreement, as the United States vowed the Islamic Republic will never acquire an atomic weapon.

Tehran threatened on Monday to restart deactivated centrifuges and ramp up its enrichment of uranium to 20% purity as its next potential big moves away from the agreement that Washington abandoned last year.

The latest war of words came the same day that Iran began enriching uranium to 4.5%, breaking the limit set in the 2015 agreement sealed under former president Barack Obama.

US Vice President Mike Pence said the international accord simply delayed Iran from obtaining a nuclear weapon by "roughly a decade", and gave away billions in economic relief that Iran could then use to wage "terrorist" attacks.

The United States "will never allow Iran to obtain a nuclear weapon", Pence told a pro-Israel Christian organisation on Monday.

"Iran must choose between caring for its people and continuing to fund its proxies who spread violence and terrorism throughout the region and breathe out murderous hatred against Israel," he said.

Pence added US sanctions have succeeded in "cutting off" Iran's ability to support armed groups in the Middle East, but he also alleged the Islamic Republic had increased its "malign activity and violence in the region" over the past several months.

Tensions in the region have risen in recent weeks after oil tankers were attacked near the Strait of Hormuz and Iran downed an unmanned US military surveillance drone.

The drone shootdown nearly led to a US military strike against Iran. It was called off at the last minute by US President Donald Trump.

The US has sent thousands of troops, an aircraft carrier, nuclear-capable B-52 bombers, and advanced fighter jets to the Middle East.

"Let me be clear," Pence said. "Iran should not confuse American restraint with a lack of American resolve."

A dying deal

Iran's nuclear programme restart threats - made by Tehran's nuclear agency spokesperson Behrouz Kamalvandi - 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.

That could raise serious questions about whether the nuclear deal, intended to block Iran from making a nuclear weapon, is still viable.

The two threats would reverse major achievements of the agreement, although Iran omitted important details about how far it might go to returning to the status quo before the pact.

Enriching uranium up to 20% purity would be a dramatic move, since that was the level Iran achieved before the 2015 deal, although back then it had a far larger stockpile.

It is considered an important intermediate stage on the path to obtaining the 90% pure fissile uranium needed for a bomb.

One of the main achievements of the deal was Iran's agreement to dismantle its advanced IR-2M centrifuges, used to purify uranium. Iran had 1 000 of them installed at its large Natanz enrichment site before the deal. Under the deal, it is allowed to operate only up to two for testing.

Still, the threatened measures also appear intended to be sufficiently ambiguous to hold back from fully repudiating the deal.

Kamalvandi did not specify how much uranium Iran might purify to the higher level, nor how many centrifuges it would consider restarting.

Iran has said all the steps it is contemplating are reversible.

Emergency diplomacy

Trump on Monday spoke to French President Emmanuel Macron about Iran's threat to ramp up enrichment of uranium.

190708060927304

"They discussed ongoing efforts to ensure that Iran does not obtain a nuclear weapon and to end Iran's destabilising behaviour in the Middle East," a White House spokesperson said in a statement.

Macron's top diplomatic advisor will travel to Iran on Tuesday and Wednesday to try to de-escalate tensions between Tehran and the United States, a presidential official said.

The French official said both Iran and the US had an interest in raising pressure at this stage, but both sides would want to start talks eventually.

"The important thing in a crisis situation such as this one is to find the middle points that take us from extreme tension to negotiation, that's what we're trying to do," the official said.

GET THE NEWS at your fingertips and download the News24 app for Android here now. Get it for your iPhone here.