Iran on Monday began enriching uranium to 4.5 per cent, just breaking the limit set by its nuclear deal with world powers, while still seeking a way for Europe to help it bypass U.S. sanctions amid heightened tensions between Tehran and Washington.

The International Atomic Energy Agency, the UN nuclear watchdog, confirmed Iran surpassed the enrichment threshold. The Vienna-based IAEA didn't specify how much beyond the 3.67 per cent threshold Iran is enriching uranium.

Uranium enriched to 90 per cent is considered weapons-grade.

Iran has threatened to restart deactivated centrifuges and sharply step up its enrichment of uranium to 20 per cent purity as its next potential big moves away from the agreement that Washington abandoned last year.

The threats, made Monday by the spokesperson for Iran's nuclear agency, would go far beyond the small steps Iran has taken in the past week to nudge its stocks of fissile material just beyond the limits in the nuclear pact.That could raise serious questions about whether the agreement, intended to block Iran from making a nuclear weapon, is still viable.

Behrouz Kamalvandi, spokesperson for Iran's Atomic Energy Organization, confirmed an announcement that Tehran had enriched uranium beyond the 3.67 per cent purity the deal allows, passing 4.5 per cent, according to the INSA news agency. That followed an announcement a week ago that it had amassed a greater quantity of low-enriched uranium than permitted.

Iran has said it will take another, third step away from the deal within 60 days, but has so far held back from formally announcing what that next step would entail.

Kamalvandi said the authorities were discussing options that included the prospect of enriching uranium to 20 per cent purity or beyond, and restarting centrifuges that were dismantled as one of the deal's core aims. (A centrifuge separates liquids or gases of different weights or densities. In nuclear engineering, they are used to separate isotopes to enrich uranium for nuclear energy or weapons.)

"There is the 20 per cent option and there are options even higher than that, but each in its own place," Kamalvandi said, according to state television. "Restarting IR-2 and IR-2 M centrifuges is an option."

Such threats from Iran will put new pressure on European countries, which insist Iran must continue to comply with the agreement even though the United States is no longer doing so.

U.S. President Donald Trump spoke to French President Emmanuel Macron about Iran's threat, the White House said.

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

A report to member states that was obtained by Reuters said the IAEA had verified the enrichment level using online enrichment monitors, and samples had been taken on Monday for analysis.

Reducing compliance

Iran also said the last chance for saving its nuclear deal with world powers will pass after a 60-day deadline.

Iranian Foreign Ministry spokesperson Abbas Mousavi told reporters Monday that Iran won't offer any further "deadlines" to save the deal by September.

Iran is pressuring European partners to find a way around U.S. sanctions and deliver the deal's promised economic relief. The U.S. unilaterally withdrew a year ago.

The nuclear agreement guaranteed Iran access to world trade in return for accepting curbs on its program. Iran says the deal allows it to respond to the U.S. breach by reducing its compliance, and it will do so every 60 days.

"If signatories of the deal, particularly Europeans, fail to fulfil their commitments in a serious way, the third step will be stronger, more decisive and a bit surprising," Mousavi said on Monday.

The new U.S. measures in place since May are intended to bar Iran from all oil exports and have succeeded in effectively pushing Iran out of mainstream oil markets.

European countries do not directly support the U.S. sanctions, but have been unable to come up with ways to allow Iran to avert them. Britain, one of Washington's main European allies, was drawn deeper into the confrontation last week when it seized an Iranian tanker it says was bound for Syria, in violation of separate EU sanctions on Syria.

U.S. tensions

Washington has imposed sanctions that eliminate any of the benefits Iran was meant to receive in return for agreeing to the curbs on its nuclear program. The confrontation has brought the U.S. and Iran close to the brink of conflict.

Enriching uranium up to 20 per cent purity would be a dramatic move, since that was the level Iran had achieved before the deal was put in place. It is considered an important intermediate stage on the path to obtaining the 90 per cent pure fissile uranium needed to make a bomb.

One of the main achievements of the deal was Iran's agreement to dismantle its advanced IR-2M centrifuges, which are used to purify uranium. Iran had 1,000 of them installed at its large enrichment site at Natanz before the deal was reached.

Under the deal, it is allowed to operate only up to two centrifuges for mechanical 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. He did not mention other more advanced centrifuges, including the most advanced, the IR-8.