French envoy in Iran talks as Trump threatens to up sanctions

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.

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."