The United States on Wednesday welcomed Iran's recent efforts to alleviate concerns about its nuclear programme, but urged it to increase the pace of cooperation.
A recent report by the UN atomic watchdog IAEA found Iran was sticking to its agreements with the agency and implementing all newly agreed measures, even addressing matters related to bomb-making for the first time in six years.
"You can't see steps taking place and say it's not sufficient, those are good steps," the US delegate to the IAEA, Ambassador Joseph Macmanus, told journalists Wednesday on the sidelines of an IAEA board of governors meeting.
"This is a long stairway to climb and each step is progress."
Addressing member states earlier, Macmanus however highlighted that Iran's engagement was "long overdue," according to a copy of his address.
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"It is only a first step," he said of Iran's compliance with recent agreements, urging "accelerated progress" and "full cooperation" on other issues, including the scaling-down of uranium enrichment and sharing of information on weapons-related issues.
Western powers fear Iran may be seeking a nuclear bomb under the cover of its civilian programme, but Tehran insists its intentions are solely peaceful.
Talks between Iran and six world powers are aiming for a potentially historic nuclear deal by July 20 -- a deadline that commentators have increasingly said will not be reached.
Macmanus however insisted that the parties were sticking to their ambitious goal.
"That's what we're still focused on," he said.
The last so-called P5+1 rounds of talks in mid-May -- between the five permanent members of the UN Security Council and Germany on one side, and Iran on the other -- ended with no apparent progress on a deal.
A next round is planned for June 16-20 in Vienna.