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Parties to the nuclear agreement are expected to resume their negotiations on Tuesday in to revive the tattered accord.

The remaining partners to the 2015 deal have been engaged in talks since early this month to try to return the United States to the accord.

Delegates from Britain, China, France, , Iran and Russia are to meet from 3:00 pm (1300 GMT) in the third round of the EU-chaired negotiations in a Vienna luxury hotel, the EU said in a statement.

Iran has refused to negotiate with the US directly, but US delegates are staying in an adjacent luxury hotel and are being regularly updated by the EU negotiator in a round of shuttle diplomacy.

The 2015 accord aimed to give Iran sanctions relief in exchange for it curtailing its nuclear programme, but the deal started to unravel in 2018 when then US president Donald Trump walked out of it.

In retaliation to the US re-imposing sanctions, Iran has started to step up its nuclear activities since 2019.

Tehran has insisted on its readiness to return to its nuclear commitments once it can assure sanctions relief.

US President Joe Biden is seeking to revive the agreement.

Negotiators have lauded progress in the talks, but also said that there is still a long way to go and details need to be worked out to save the accord.

EU negotiator Enrique Mora said in a column published Monday on the Spanish site Politica Exterior that "many obstacles" remained, mentioning "domestic politics in Tehran and Washington, where the agreement probably has more detractors than supporters".

The hope is to achieve a concrete result "by the end of May", before Iranian presidential elections in June, a diplomat familiar with the discussions told AFP.

Last Thursday, Iran's foreign ministry issued a defence of its negotiating team, following days of growing criticism by state media.

A senior US official last week said Washington had shared details of the sanctions it was prepared to lift with Tehran.