VIENNA - A senior U.N. official announced progress Friday in long-stalled efforts to resume a probe of suspicions that Iran may have worked to develop nuclear arms, saying the investigation could be restarted by early next year.

Herman Nackaerts of the International Atomic Energy Agency spoke of headway after returning from talks with Iranian officials in Tehran, saying his team was "able to make progress" and the IAEA expects to "start implementing" an agreement on the probe shortly after new talks in Tehran on Jan. 16.

He acknowledged that a key IAEA request - a visit to a site linked to suspected nuclear weapons research - was again denied. But he told reporters at Vienna airport that he expected IAEA experts to be allowed to go to Parchin, a military base southeast of Tehran, and follow up on suspicions it might have been used as part of secret arms-related experiments.

In Tehran, Iran's state TV also reported that there was no agreement on visiting Parchin. But in comments to state TV, Iranian envoy Ali Asghar Soltanieh called the talks "constructive," adding that "good progress was made." He also said a new meeting was set for Jan. 16.

While Nackaerts' comments gave cause for some optimism, they were not the first instance of a senior IAEA official saying the standoff was close to resolution.

Iran says it does not want atomic arms and has justified a nearly yearlong delay in cooperating with the IAEA's probe by saying a framework regulating such an investigation must be agreed on first. But as talks on such an agreement have dragged on, agency officials have complained that they might be just a delaying tactic.

They are particularly concerned that such delays can hurt their efforts to investigate Parchin. The IAEA suspects that Iran has conducted live tests of conventional explosives there that could be used to detonate a nuclear charge and cited satellite photos indicating a cleanup of the site at the large military base.

Iran denies it is sanitizing the site, but IAEA chief Yukiya Amano has warned that his agency's chances of a meaningful investigation there are diminishing.

Nackaerts offered no details on the substance of his talks. But diplomats familiar with the negotiations have said previously that the agreement was stalled because the IAEA wanted repeated access to sites, officials or documents of interest. Instead, Tehran demanded that once such access was granted, the person, document or site be off-limits to the IAEA for repeated visits, questioning or viewing.

In Washington, U.S. State Department spokesman Patrick Ventrell commended the IAEA for its efforts, but said, "We're disappointed that Iran did not grant access to the Parchin site, which Iran has been sanitizing in advance of re-engaging the IAEA."

Regarding the meeting planned for January, Ventrell said, "We hope that Iran starts the immediate substantive cooperation that is long overdue."