"We do not see anything that would give rise to hypotheses of any substantive work going on there," IAEA Director General Rafael Grossi said in New York.
"These are big industrial sites where there is movement, there is activity going on and we are very quick to indicate that this does not imply that there is activity on enrichment," he added.
Iran suspended cooperation with IAEA inspectors after a 12-day war in June against Israel and the United States, codified via a new law passed by parliament.
Grossi told reporters that inspectors had no access to the to sites stricken in June, but confirmed that some inspection was under way.
"We are trying to build it back, and we are inspecting in Iran," he said, "not at every site where we should be doing it - but we are gradually coming back."
Respecting NPT
In September, Iran and the agency agreed in Cairo to restart inspections. However, after Germany, France and the United Kingdom triggered the reimposition of UN sanctions, it remained unclear whether Iran would comply.
Foreign Ministry spokesman Esmaeil Baghaei said on Tuesday that Iran's commitments under the Non-Proliferation Treaty (NPT) safeguards agreement with the agency remain in place.
"In fulfilling these safeguards obligations, we are maintaining interactions with the IAEA while taking into account parliament's law, which designates the Supreme National Security Council (SNSC) as the authority responsible for decisions on cooperation with the agency," official media cited Baghaei as saying. 
Under the new law, inspections proceed via SNSC approval for limited IAEA access at non-stricken sites, maintaining safeguards obligations under conditional cooperation while excluding full reporting and NPT oversight.
The Center for Strategic and International Studies (CSIS) reported on Tuesday citing satellite imagery that Iran has continued construction at a major underground nuclear site near Natanz.