Director-General of the International Atomic Energy Agency (IAEA) Rafael Mariano Grossi addresses the media during a news conference behind plexiglass shields after a meeting of the IAEA board of governors at the International Center in Vienna, Austria, March 1, 2021. (RONALD ZAK / AP)

VIENNA - The International Atomic Energy Agency (IAEA) is deeply concerned about unreported nuclear material in Iran, said the agency's chief on Monday.

The presence of multiple uranium particles of anthropogenic origin is a clear indication that nuclear material and/or equipment contaminated by nuclear material has been present at a location not declared by Iran, Director-General Rafael Grossi said at an IAEA board of governors online meeting.

"In the absence of a technically credible explanation from Iran, the agency is deeply concerned that undeclared nuclear material may have been present at this undeclared location and that such nuclear material remains unreported by Iran under its Safeguards Agreement," he said.

Iran has not answered any of the IAEA's questions relating to the possible presence of nuclear material at three other locations either, said IAEA Director-General Rafael Grossi

ALSO READ: Khamenei: Iran could increase uranium enrichment to 60%

Iran has not answered any of the IAEA's questions relating to the possible presence of nuclear material at three other locations either, he added.

Grossi told reporters after the meeting that his agency has lost the Additional Protocol and "the transparency measures specific to the JCPOA (Joint Comprehensive Plan of Action)".

"To give you an example, if I wanted to go to a place which is part of a declared site, of which I have doubt, I cannot," he said. "It is a huge loss."

He emphasized that the agreement he reached with Iran on Feb 21 is "a temporary technical understanding and that it is compatible with Iranian law".

"It is to enable the agency to resume its full verification and monitoring of Iran's nuclear-related commitments under the JCPOA if and when Iran resumes its implementation of those commitments," he said.

READ MORE: Iran, IAEA make temporary pact for 3-month inspection

He told reporters that the US president and secretary of state have both indicated that "their aspiration is to return to the JCPOA," and are "looking into means and ways to do it," noting it is important to give more time to diplomacy and consultations.

"This is going to take time," he said. "I hope they will agree and we are going to be supporting them."

The P5+1 countries (China, France, Russia, Britain and the United States, plus Germany) negotiated the JCPOA on the Iranian nuclear program in 2015, before Washington under former US President Donald Trump withdrew from the nuclear agreement in 2018.