SEOUL, South Korea — The head of the U.N.’s nuclear watchdog said Wednesday that “very detailed” measures to verify Iran’s nuclear activities must be included in a potential U.S.-Iran agreement to end their war in the Middle East.

International Atomic Energy Agency Director General Rafael Grossi stressed the need for the thorough verification regime for Iran’s nuclear program, as U.S. President Donald Trump said Tuesday that a second round of talks with Iran could happen over the next two days.

The Trump administration has said that preventing Iran from gaining a nuclear weapon is a key war aim. Iran has previously said it isn’t developing such weapons but rejected limits on its nuclear program.

Last weekend in Pakistan, an initial round of talks between the two countries failed to produce an agreement. The White House said Iran’s nuclear ambitions were a central sticking point. But an Iranian diplomatic official, speaking on the condition of anonymity because of the sensitivity of the closed-door talks, denied that negotiations had failed over Iran’s nuclear ambitions.

“Iran has a very ambitious, wide nuclear program so all of that will require the presence of IAEA inspectors,” Grossi told reporters in Seoul. “Otherwise, you will not have an agreement. You will have an illusion of an agreement.”

He said that any agreement on nuclear technology “requires very detailed verification mechanisms.”

Iran has not allowed the IAEA access to its nuclear facilities bombed by Israel and the United States during a 12-day war in June, according to a confidential IAEA report circulated to member states and seen by The Associated Press in February.

The report stressed that it “cannot verify whether Iran has suspended all enrichment-related activities,” or the “size of Iran’s uranium stockpile at the affected nuclear facilities.”

Iran has long insisted its program is peaceful, but the IAEA and Western nations say Tehran had an organized nuclear weapons program up until 2003.

The IAEA has maintained Iran has a stockpile of 440.9 kilograms (972 pounds) of uranium enriched up to 60% purity, a short, technical step away from weapons-grade levels of 90%.

That stockpile could allow Iran to build as many as 10 nuclear bombs, should it decide to weaponize its program, Grossi said earlier.

Such highly enriched nuclear material should normally be verified every month, according to the IAEA’s guidelines.