In such a scenario, Iran could "restart its activities without any limits," Iran's ambassador to the International Atomic Energy Agency (IAEA) Reza Najafi told reporters on the sidelines of a meeting of the IAEA's board in Vienna.

The future of the deal -- known as the Joint Comprehensive Plan of Action (JCPOA) -- has been thrown into doubt after President Donald Trump announced last month that the US would withdraw from the accord and re-impose sanctions.

The preparatory works mentioned by Najafi refer to steps to boost uranium enrichment capacity by producing new centrifuges, as outlined on Tuesday by Vice President Ali Akbar Salehi, who heads the Iranian Atomic Energy Organisation.

Najafi said that in addition Iran had notified the IAEA of a plan to restart activity at its uranium conversion facility in Isfahan to produce the UF6 feedstock for centrifuges.

The IAEA confirmed on Tuesday that it had "received a letter from Iran on 4 June informing the Agency that there is a tentative schedule to start production of UF6".

However, Najafi emphasised that the measures do "not mean that right now Iran will start any activities contrary to the JCPOA".

The remaining signatories to the pact have been scrambling to find ways to preserve the accord since Trump's announcement.

Last month a senior Iranian official said European powers had until the end of May to come up with an economic package to compensate Iran for the effects of the US withdrawal.

On Wednesday Najafi said that negotiations were still continuing at an expert level but that it could not be an "endless process" and needed to be concluded "very soon".

Asked about the IAEA's call for Iran to provide "timely and active cooperation" with the inspections mandated under the JCPOA, Najafi said that Iran had interpreted this as encouragement to extend voluntary invitations to the agency to conduct inspections.

However he said that "while Iran is not benefiting from the deal, no one should expect Iran to implement... voluntary measures".