Emmanuel Bonne pressed the high-level talks with Iranian President Hassan Rouhani

French President Emmanuel Macron's top diplomatic advisor met with Iran's president Wednesday winding up a day of talks in Tehran aimed at saving a landmark 2015 nuclear deal and easing tensions between Tehran and Washington.

But as Emmanuel Bonne pressed the high-level talks, President Donald Trump took to Twitter to warn that US sanctions against Iran would soon be "increased substantially", charging Tehran had "long been secretly 'enriching'" uranium.

The 2015 accord between Iran and world powers, the Joint Comprehensive Plan of Action (JCPOA), promised sanctions relief, economic benefits and an end to international isolation in return for stringent curbs on the Islamic republic's nuclear programme.

Iran has long been secretly "enriching," in total violation of the terrible 150 Billion Dollar deal made by John Kerry and the Obama Administration. Remember, that deal was to expire in a short number of years. Sanctions will soon be increased, substantially! - Donald J. Trump (@realDonaldTrump) July 10, 2019

But Tehran says it has lost patience with perceived inaction by European countries more than a year after Trump unilaterally pulled the US out of the agreement and started to impose punishing sanctions.

In his meeting with Bonne, Iran's President Hassan Rouhani said Tehran had "completely kept the path of diplomacy and talks open", according to a statement from his office.

He called on other parties to the deal to "completely implement their commitments" to keep it alive.

Bonne also met Rear-Admiral Ali Shamkhani, the secretary of Iran's Supreme National Security Council, and Foreign Minister Mohammad Javad Zarif and his deputy Abbas Araghchi.

His mission was "to try and open the discussion space to avoid an uncontrolled escalation, or even an accident", according to French Foreign Minister Jean-Yves Le Drian.

Before meeting with Bonne, Zarif said "negotiations are never possible under pressure", in reference to US sanctions against Iran.

Pointing to the US withdrawal from the JCPOA, he added that the Europeans "must solve that problem."

'Nuclear extortion'

Bonne arrived in Tehran after Iran announced on Monday it had surpassed 4.5 percent uranium enrichment -- above the 3.67 percent limit under the accord, though still far below the 90 percent necessary for military purposes.

Earlier this month, it was confirmed that Iran surpassed 300 kilogrammes of enriched uranium reserves, another limit that was imposed by the deal.

At Washington's request, the UN's nuclear watchdog held a special meeting Wednesday at its Vienna headquarters.

US delegate Jackie Wolcott told the gathering that Iran was engaged in "nuclear extortion".

Her Iranian counterpart Kazem Gharib Abadi hit back, calling it a "sad irony" that the meeting was convened at Washington's request and claiming the current standoff was a result of the US's "outlaw behaviour".

Russia's ambassador to the International Atomic Energy Agency, Mikhail Ulyanov, later tweeted that the US "was practically isolated on this issue".

In a joint statement, European parties to the deal Germany, France and Britain said their continued support for the accord "relies on Iran implementing its commitments".

But they added the "issues at hand should be addressed by participants to the JCPOA".

Ahead of the meeting, a source at the French presidency said "we are in a very critical phase. The Iranians are taking measures that are in violation (of the agreement) but (they) are very calibrated".

Iran ends 'strategic patience'

After Washington withdrew from the JCPOA in May 2018, it reimposed stinging sanctions on Tehran, hitting the banking and oil sectors hard.

As the Iranian economy went into free fall, Iran demanded that the other parties -- especially the Europeans -- deliver promised economic benefits and help it bypass the US sanctions.

However, it became clear that this was no simple task, and Iran -- whose economy depends heavily on oil exports -- changed tack and indicated it would reshape its policy of "strategic patience".

In May, a year after Trump's withdrawal, Rouhani said Iran would roll back its commitments under the deal in stages every 60 days in an effort to force the other parties to deliver on their side of the bargain.

As tensions rose, the US dispatched a naval carrier, bombers and extra troops to the region to counter perceived threats from Iran.

Last month, Trump said he had called off a retaliatory military strike against Iran at the last minute after the Islamic republic shot down a US drone that it said had crossed into its airspace, a claim denied by Washington.

Trump re-upped the pressure Wednesday, claiming "Iran has long been secretly 'enriching'" uranium in violation of the accord.

"Remember, that deal was to expire in a short number of years. Sanctions will soon be increased, substantially!"