The closed-door encounter involving representatives from Iran, Britain, China France, Germany and Russia as well as the European Union had long been on the cards, but comes as tensions rise after Tehran announced a plan to boost uranium enrichment capacity.

"It is one of the technical meetings that are held regularly," a diplomatic source told AFP, adding that it would focus on economic issues.

The remaining parties to the landmark 2015 deal are scrambling to save the agreement after US President Donald Trump abandoned it last month.

The US move to reimpose sanctions -- lifted under the accord that aims to ensure Tehran will not develop a nuclear bomb -- has already seen many investors who returned to the country start to wind down operations.

Iran raised pressure on European diplomats seeking to rescue the pact by announcing Tuesday it has launched a plan to boost uranium enrichment capacity with new centrifuges.

The European Union said a first assessment indicated the new steps did not constitute a violation of the agreement, but warned they did "not contribute to building confidence".

French Foreign Minister Jean-Yves Le Drian warned Iran on Wednesday that its plan to increase its uranium enrichment capacity took it close to a "red line".

Iran insists its nuclear programme is for civilian uses only, but opponents in the United States, Israel and Saudi Arabia accuse it of seeking to build an atomic bomb.