Talks in Vienna, Austria, to revive the 2015 Iran nuclear deal are moving ahead slowly as negotiators discuss sanctions relief and controls on Tehran's nuclear programme.
While the US has been cautious in its comments, saying it is too soon to tell how the talks are progressing, Iran and Russia have expressed optimism about the possibility of an agreement.
Mikhail Ulyanov, Moscow's ambassador at the talks, said he had two "useful" meetings with Washington's Iran envoy, Rob Malley, on Wednesday evening to discuss ways of restoring the pact, known as the Joint Comprehensive Plan of Action.
Negotiators indicated before the start of the eighth round of discussions this week that the process would end in January. Mr Ulyanov, responding to a question on Twitter, suggested that talks could be finished by early February.
US State Department spokesman Ned Price said that Secretary of State Antony Blinken discussed the negotiations in phone calls with the foreign ministers of France, Germany and Britain.
The three countries are signatories to the JCPOA, along with the US, Russia and China.
"The secretary and his counterparts also discussed their shared concerns about the pace of developments in Iran’s nuclear programme as time runs short for Tehran to return to the JCPOA," Mr Price said.
The original deal fell apart in 2018 after then US president Donald Trump abandoned it and reimposed sanctions on Iran's economy.
Mr Trump's decision put the US in breach of the accord and has since been described by foreign policy analysts and Mr Blinken as a big mistake.
Iran continued its compliance with the agreement for a time after the US' exit, but then began exceeding limits on its enrichment of uranium and other provisions of the accord.