Britain, France, Germany, China and Russia are struggling to save the landmark 2015 accord with Iran, which has been progressively stepping up its nuclear activities since the United States pulled out of the deal in 2018.

Tehran insists it is entitled to do so under the terms of the accord -- which swapped sanctions relief for Iran's agreement to scale back its nuclear programme -- following Washington's withdrawal and reimposition of sanctions.

EU senior official Helga Schmid, who chaired the talks in Vienna on Tuesday, wrote on Twitter that the meeting's participants were "united in resolve to preserve the #IranDeal and find a way to ensure full implementation of the agreement despite current challenges".

Representatives from Britain, China, France, Germany, Iran and Russia all attended the talks -- part of a regular series of gatherings to discuss the accord, which have been increasingly tense since the US pullout began unravelling the agreement.

- 'Mockery' -

China's representative, senior Foreign Ministry official Fu Cong, told reporters after the meeting that Iran needed to come back to full compliance, but at the same time "the economic benefit that is due to Iran needs to be provided".

He slammed the US for "making a mockery of international law" in its "attempt to sabotage and to kill the JCPOA", referring to the abbreviation of the deal's formal name, the Joint Comprehensive Plan of Action.

"The US is stopping at nothing in trying to sabotage other countries' efforts to provide economic benefits to Iran," he added.

The United Nations last week blocked the US bid to reimpose international sanctions on Iran, while Washington also failed to rally enough support to extend an arms embargo that was scheduled to start being rolled back from October.

In a boost to Tuesday's talks, the Iranian atomic energy agency last week also agreed to allow inspectors of the UN nuclear watchdog to visit two sites suspected of having hosted undeclared activity in the early 2000s.

International Atomic Energy Agency head Rafael Grossi had travelled to Iran on his first trip since taking up the top post last year and after months of calling for access.

- US 'isolated' -

Mark Fitzpatrick, an associate fellow of the International Institute for Strategic Studies (IISS), said last week's agreement on access kept "Iran generally in line with the rest of the world, against an isolated United States".

But Fitzpatrick pointed out that "Iran's nuclear activities remain of deep concern to those states that are dedicated to non-proliferation".

Iran reportedly recently transferred advanced centrifuges used to enrich uranium from a pilot facility into a new hall at its main Natanz nuclear fuel plant, which was hit by sabotage in July.

An IAEA assessment published in June said Iran's stockpile of enriched uranium was almost eight times the limit fixed in the accord.

The level of enrichment is still far below what would be needed for a nuclear weapon, but parties to the deal have urged Iran's full compliance.

Iran has insisted it can reverse the steps it has taken since last year -- if it can again benefit economically again under the deal.

The IAEA, which regular updates its members on Iran's nuclear activities, is expected to issue a fresh report ahead of a meeting of member states to discuss the dossier later this month.