Yukiya Amano, head of the U.N.'s International Atomic Energy Agency, said his meetings with Iranian President Hassan Rouhani resulted in a "better understanding on some ways forward," but that "more work will be needed."
The formulation of his statement was similar to previous ones issued by the IAEA, which has struggled for nearly a decade to resolve its concerns.
Amano's trip Thursday to Tehran was significant because it represented his last chance to secure access and cooperation before a July 7 deadline for a long-term nuclear agreement between Iran and world powers.
Rouhani also provided no hint of substantial progress. Iran has previously acknowledged some activities like experiments with detonators, but says those activities had no connection to exploding a nuclear device and were instead developed for industrial purposes.
Repeating the standard Iranian line, Rouhani said after meeting Amano that the agency now understands the "pointless allegations" are "baseless."
The issue was put on the IAEA front burner four years ago when the agency published an annex of 12 alleged activities it said pointed to nuclear weapons research and development by Iran.
A U.S. intelligence assessment published in 2007 raised similar allegations, but said the work ended early last decade. Iran says the suspicions are based on doctored intelligence from Israel, the United States and other adversaries.
The U.N. agency's investigation has gained even more significance as part of the talks that are ongoing in Vienna to crimp Iran's nuclear program for the next decade. At least part of the sanctions relief for Iran under any pact will depend on its full cooperation with the IAEA.
U.S Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif have taken the lead in the negotiations. In comments echoed by Zarif ahead of their renewed meeting Friday evening, Kerry said the talks "are making progress." But he also spoke of "some tough issues," telling reporters, "We have a lot of work to do."
Amano said he discussed his agency's monitoring of Iran's commitments under any deal. Backed by the U.S., the agency seeks pervasive oversight to ensure Tehran doesn't cheat.
But a senior Iranian negotiator on Thursday rejected any extraordinary inspection rules.
Speaking to reporters in Austria's capital, the official said the IAEA's standard rules governing access to government information, sites of interest and scientists should be sufficient to ensure that Iran's program is solely for peaceful purposes.
Iran has committed to implementing the IAEA's "additional protocol" for inspections and monitoring as part of an accord. The protocol gives the IAEA expanded access to declared and potentially undeclared nuclear sites, and to the sensitive information of the more than 120 governments that accept its provisions.
But the rules don't guarantee monitors can enter any facility they want to and offer no specific guidance about sensitive military sites — an issue of particular interest with Iran, given the long-standing allegations of secret nuclear weapons work.
Negotiators have given themselves until at least July 7 to reach agreement.