The United States said on Tuesday it had seen possible progress in talks with Iran, but joined European negotiators in pressing for urgency in rolling back Tehran's nuclear program, AFP reports.
"There may have been some modest progress," State Department spokesman Ned Price told reporters in Washington, adding, "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."
The indirect talks between the US and Iran on reviving the 2015 nuclear agreement resumed in Vienna on Monday.
Current President Joe Biden supports a return to the deal but Iran has kept taking steps away from compliance as it presses for sanctions relief.
The negotiations to salvage the agreement restarted in late November, after a five-month hiatus following the election of Iranian President Ebrahim Raisi.
The talks involve delegations from Iran and the other countries that remain party to the deal -- Britain, China, France, Germany and Russia.
Washington is participating indirectly, with diplomats shuttling back and forth between the Iranian and the US sides.
Following Monday’s session, the European Union diplomat in charge of the nuclear talks warned of "difficult" work ahead.
The envoy, Enrique Mora, said all sides were showing "a clear will to work toward the successful end of this negotiation".
"It's a very good sign... We will be working very seriously in the days and weeks ahead... It's going to be very difficult," Mora told reporters.
The Islamic Republic has repeatedly demanded that the US lift sanctions imposed on Iran and also reassure Iran it will not abandon the deal again as a precondition for its returning to compliance with the deal.
US officials have said that while they prefer the diplomatic route to reach an agreement with Iran, there are other options on the table should that fail.