UN nuclear watchdog chief Rafael Grossi earlier said he had held "constructive" talks with Iranian officials in Tehran after the discovery of uranium particles enriched to near weapons-grade level.

On his return to Vienna, Grossi recalled there had been "a reduction in monitoring activities related to cameras and monitoring systems" and said "we have agreed that those will be operating again".

"This is very, very important" in terms of continuity of knowledge, "in particular in the context of the possibility of the revival of JCPOA", he said.

Grossi arrived in Iran on Friday talks deadlocked on reviving a landmark 2015 accord on Iran's nuclear activity, known formally as the Joint Comprehensive Plan of Action, or JCPOA.

"We have put a tourniquet on the bleeding of information and lack of continuity of knowledge we had -- now we can start working again. These are not words, this is very concrete," he said.

The Vienna-based IAEA has been seeking greater cooperation with Iran over its nuclear activities.