UN nuclear watchdog chief Rafael Grossi said there was a "step forward" in talks between Iran and the United States in Geneva but warned "we don't have much time".
The two sides took part in Omani-mediated talks in the Swiss city this week, with US threats of military action looming over the negotiations.
In an interview with French TV channel LCI, Grossi, the head of the International Atomic Energy Agency said "we made progress, but there is still work to be done, and the problem is that we don't have much time".
"There is the possibility of a dialogue that is really beginning, for the first time, to take shape," he said.
"We are starting to talk about concrete things, about what we have to do," the Argentinian diplomat added.
He said he saw "a willingness on both sides to reach an agreement, but obviously such an agreement is enormously complex".
Tehran has suspended some cooperation with the IAEA and prevented the watchdog's inspectors from accessing sites bombed by Israel and the United States during a 12-day war last June, accusing the UN body of bias and of failing to condemn the strikes.
The White House has warned Iran that it would be "wise" to do a deal, while US President Donald Trump again hinted at military action against the longtime American foe.
Mr Trump first threatened military action against Iran over the authorities' deadly crackdown on anti-government protests in January.
While talks between the two sides have now resumed for the first time since the 12-day war collapsed the last set of negotiations, Washington has continued to build up its military forces in the Middle East.
Iran expected to submit written proposal for resolving standoff with US, senior US official says
Iran is expected to submit a written proposal on how to avoid its standoff with the United States in the wake of US-Iran talks in Geneva on Tuesday, a senior US official said.
Top national security advisers met in the White House Situation Room to discuss Iran and were told all US forces deployed to the region should be in place by mid-March, the official said.
Secretary of State Marco Rubio will meet Israeli Prime Minister Benjamin Netanyahu in Israel on the weekend of 28 February, the official said.
Read our latest Middle East stories here
