The head of the U.N.’s nuclear watchdog will go to Tehran to meet with Iranian authorities and urge them to grant the agency access to a number of suspected sites of undeclared nuclear material, the agency announced Saturday.

The International Atomic Energy Agency (IAEA)’s Director General Rafael Grossi, who took the position in December, will make his first visit to Iran on Monday at a time as the U.S. is ramping up pressure on Tehran and calling for the international community to get tougher against the regime’s nuclear ambitions.

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“I have decided to come personally to Tehran so that I can reinforce the importance of cooperation and the full implementation of all safeguards commitments and obligations with the IAEA,” Grossi said in a statement.

“My objective is that my meetings in Tehran will lead to concrete progress in addressing the outstanding questions that the Agency has related to safeguards in Iran and, in particular, to resolve the issue of access," he said.

The access requested by the IAEA is focused on two sites from before Iran signed the 2015 nuclear deal -- formally known as the Joint Comprehensive Plan of Action (JCPoA). It is thought that those sites contained undeclared nuclear material.

The Trump administration took the U.S. out of the deal in 2018, and has reimposed a number of sanctions on Iran as part of a "maximum pressure" campaign. This week, it triggered a process contained in the U.N. Security Council Resolution 2231 that will “snapback” all U.N. sanctions in Iran, including a soon-to-expire arms embargo that the Council failed to extend last week.

Both U.S. allies and adversaries have claimed that, because the U.S. left the deal, it has no legal basis to invoke snapback. But the U.S. argues that while it left the deal, it still retains rights under Resolution 2231. The move sets a 30-day countdown until the sanctions are re-imposed.

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At the U.N. on Friday, Secretary of State Mike Pompeo cited the difficulties the IAEA is having in getting Iran to give it access.

“This is happening, of course, at a moment when Iran is refusing to allow the IAEA to inspect sites suspected of undeclared nuclear activity, that were formerly part of Iran’s nuclear weapons program,” he said.

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He also took direct aim at the United Kingdom, France and Germany for not supporting the extension, while accusing them of privately telling him they didn’t want the embargo lifted.

“And yet today, in the end, they provided no alternatives, no options. No country but the United States has had the courage and conviction to put forward a resolution,” he said. “Instead they chose to side with the ayatollahs. Their actions endangered the people of Iraq, of Yemen, of Lebanon, of Syria and indeed their own citizens as well.”

The Associated Press contributed to this report.