The International Atomic Energy Agency has said in recent confidential reports to member states seen by Reuters that Iran has been installing and enriching with more cascades, or clusters, of advanced centrifuges at its underground enrichment plants at Natanz and Fordow.

At the same time, indirect talks with the United States on reviving a largely hollowed-out 2015 nuclear deal with Iran are stalled, with officials saying one important sticking point has been Iran's demand that the IAEA end an investigation into uranium traces found at undeclared sites.

Asked in an on-stage discussion in Washington how he sees the world today, Grossi started with Iran rather than Ukraine and said it "continues to be a problem".

"I see every day through my inspectors how this problem is getting more and more relevant, and I'm choosing a word which is neutral. It's an even more relevant problem every day," Grossi told the Carnegie International Nuclear Policy Conference, without elaborating.

He added later that he would not cave to political pressure over his investigation of the uranium traces and his efforts to obtain explanations from Iran on how they came to be there.

"I will never do anything in the verification area under political pretences or for political reasons. The IAEA has to do what it has to do. I say it here publicly and I've said it to my Iranian counterparts many times when they request that we look elsewhere."