The new head of the International Atomic Energy Agency (IAEA) has warned Iran there will be serious consequences if there is any repeat of last year’s detention of an IAEA inspector.

Speaking to journalists on his first official trip to Washington as IAEA director general, Rafael Grossi said he had met a senior Iranian official inVienna in December and expressed his concern over what he described as a “grave” incident.

“It’s something I believe I dealt with constructively and very clearly. I said: this for us is unacceptable,” Grossi said he told the Iranian deputy foreign minister, Abbas Araghchi. “I think there’s an understanding between us that this cannot be repeated.”

Grossi said that the IAEA was also talking with Iran about the discovery of uranium particles last year at a site in Turquzabad outside Tehran, and that he was hoping to get more information from an Iranian delegation due to visit IAEA headquarters in Vienna this month. He said he hoped that he would have the full backing of the international community to persuade Iran to comply.

IAEA inspectors have remained in Iran monitoring its nuclear programme, even after US withdrawal from the 2015 agreement that imposed limits on that programme and Iran’s subsequent declaration that it would begin ignoring some of those constraints. The inspectors have also continued their work despite the dramatic rise in tensions following the US assassination of the Iranian general Qassem Suleimani in January.

Grossi said that the ejection of IAEA inspectors would trigger “a major international crisis” but he also said he would withdraw the inspectors immediately if there was any threat to their safety.

Iran revoked an IAEA inspector’s accreditation in November, saying she had triggered an alarm at a uranium enrichment facility in Natanz, because she had traces of explosive chemicals on her. She was denied entry to the site, and briefly denied permission to leave Iran.

Grossi said Iran’s allegations were unfounded. The Argentinian diplomat, who took the helm at the IAEA in December, said he respected Iran had its own view of what happened at Natanz, but added: “In whatever scenario, an inspector has privileges and immunities, and these have to be respected.

“If they are not respected, that could be a serious problem so I think we have an understanding.”

Grossi insisted that the incident had not stopped inspectors continuing to monitor Iranian nuclear facilities to ensure restricted material is not diverted, but added there was an increased emphasis on safety.

“I’m responsible for these people,” he said. “They are out there, so their security and safety comes first. If I felt there’s a problem there, I will take them out immediately.”

Grossi made clear that Iran had yet to provide an explanation of the uranium traces at the site at Turquzabad, but he would continue to press for answers.

“We are in a process and any process has ups and downs,” he said.

At an appearance at the Carnegie Endowment for International Peace later on Wednesday, he said: “When I request Iran to work with us … I hope that countries will give me the necessary support that I need.”

Grossi said that he also discussed North Korea at a meeting on Tuesday with the US secretary of state, Mike Pompeo, and said that if a nuclear agreement was made with Pyongyang, the IAEA would be ready to send in an inspection team “while the ink is still fresh” to verify compliance.

“When we return, we will find a country that is completely different from the one our inspectors were kicked out of 10 years ago … It’s a country in possession of nuclear weapons.”

He added: “It’s going to be a massive, massive effort, much bigger than the one in Iran. So it’s going to be a great challenge for us, and I’m trying to make sure that we are up to the task.”