The US struck a slightly more optimistic tone on Tuesday about possible progress in talks with Iran but joined European negotiators in pressing for urgency in rolling back the country's nuclear programme.
"There may have been some modest progress," State Department spokesman Ned Price said.
“But it is in some ways too soon to say how substantive that progress may have been. At a minimum any progress, we believe, is falling short of Iran's accelerating nuclear steps and is far too slow.”
Former president Donald Trump in 2018 withdrew the US from the nuclear accord, known as the Joint Comprehensive Plan of Action (JCPOA) and reimposed sanctions on Iran.
The latest round of talks in Vienna comes after Iran elected a new conservative government.
“This negotiation is urgent,” delegates from Britain, France and Germany said.
“We are clear that we are nearing the point where Iran's escalation of its nuclear programme will have completely hollowed out the JCPOA,” said the three countries, known as the E3 powers.
“That means we have weeks, not months, to conclude a deal before the JCPOA's core non-proliferation benefits are lost.”
Iran was in compliance with the 2015 deal before Trump's exit and for some time after, but eventually stepped up its enrichment of uranium. The country denies that it wants to acquire a nuclear arsenal.
Iran 'significantly closer' to fissile material
On Tuesday, the E3 negotiators said 60 per cent enrichment was still “unprecedented for a state without nuclear weapons”. Military-grade levels are at about 90 per cent.
“Its increasing 60 per cent stockpile is bringing Iran significantly closer to having fissile material, which could be used for nuclear weapons,” they said.
The US did not specify areas of progress but Russia, which is participating along with China and the Europeans, said a working group had a “useful meeting” on nuclear issues and informal discussions on lifting sanctions.
“We observe indisputable progress,” Moscow's ambassador to the UN in Vienna, Mikhail Ulyanov, wrote on Twitter.
US negotiator Rob Malley is participating indirectly, with European diplomats shuttling between hotels, as Iran refuses direct contact with Washington.
Iran's Foreign Minister Hossein Amirabdollahian was quoted by state news agency IRNA on Tuesday as saying the negotiations were “on a good track". He said: "We think if other parties continue this round of the talks with good faith, reaching a good agreement for all parties is possible. If they show seriousness, in addition to the good faith, a deal can be reached soon and in the near future."
“With the goodwill and seriousness from the other parties, we can consider a quick agreement in the near future."
EU diplomat Enrique Mora said all sides were showing “a clear will to work towards the successful end” but that “very difficult” negotiations lay ahead.