The indirect negotiations in Doha are an attempt to reboot long-running European Union-mediated talks in Vienna on a return to the 2015 nuclear agreement between Tehran and world powers.

EU coordinator Enrique Mora said late Wednesday that "two intense days of proximity talks" in Doha had "not yet" yielded the progress that the EU team had hoped for.

"We will keep working with even greater urgency to bring back on track a key deal for non-proliferation and regional stability," he added on Twitter, posting a photo of himself meeting with Iran's chief negotiator Ali Bagheri.

The comments came after Iran's foreign ministry spokesman Nasser Kanani said that the talks would last only two days.

"The talks in Doha, which are taking place in a professional and serious atmosphere, were scheduled for two days from the beginning," Kanani said.

The parties have "exchanged views and proposals on the remaining issues", he added.

No time-limit had previously been announced on the talks, which have been taking place in a Doha hotel with special envoy Robert Malley heading the US delegation.

An EU source told AFP that the discussions, which come two weeks before US President Joe Biden makes his first official visit to the region, were supposed to last several days.

- 'Red lines' -

Foreign Minister Hossein Amir-Abdollahian had said Iran was "serious" about finalising a deal in Doha, but that it wouldn't cross its "red lines".

"If the American side has serious intentions and is realistic, an agreement is available at this stage and in this round of negotiations," he was quoted as saying by state news agency IRNA earlier Wednesday.

IRNA has previously described the "red lines" as lifting all sanctions as related to the nuclear agreement, creating a mechanism to verify they have been lifted, and making sure the US does not withdraw once again from the deal.

The US State Department said that the indirect consultations had been ongoing Wednesday in Doha, but said it had nothing immediately to say about the talks.

A State Department spokesperson said the US was prepared to return to the deal, but reiterated calls for Tehran "to drop their additional demands that go beyond" the scope of the pact.

Differences between Tehran and Washington have notably included Iran's demand that its Islamic Revolutionary Guard Corps be removed from a US terror list.

- 'Trump method' -

The arch-rivals have been meeting indirectly -- passing messages from different areas of the same hotel -- to try to break an impasse in attempts to restart the 2015 agreement.

The deal, which lifted sanctions in return for Iran curbing its nuclear programme, was abandoned unilaterally in 2018 by former US president Donald Trump, who proceeded to reimpose biting sanctions.

Iranian officials earlier said they were hoping for progress in Qatar -- but warned the Americans to abandon the "Trump method" of negotiating.

"We hope that, God willing, we can reach a positive and acceptable agreement if the United States abandons the Trump method," Iranian government spokesman Ali Bahadori-Jahromi said.

He described the method as "non-compliance with international law and past agreements and disregard for the legal rights of the Iranian people".

The international talks on reviving the deal began in April 2021 in Vienna, before the process stalled in March.

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