United Nations inspectors have found new evidence of undeclared nuclear activities in Iran, according to three diplomats briefed on the discovery, raising new questions about the scope of the country’s atomic ambitions. Samples taken from two sites during inspections in the fall by the U.N.’s International Atomic Energy Agency contained traces of radioactive material, the diplomats said, that could indicate Iran has undertaken work on nuclear weapons, based on where it was found. The diplomats said they didn’t know the exact nature of what was found.

Last year, Iran blocked IAEA inspectors from checking the sites involved for seven months, leading to a standoff. Tehran has long denied that it has sought to make an atomic bomb and said all of its nuclear work is for peaceful purposes such as power generation and health care. There was no immediate comment from Iran on the findings.

In Washington, officials at the White House and State Department didn’t immediately respond to requests for comment. In recent months, Iran has scaled up its nuclear activities, breaching many of the limits in the 2015 nuclear agreement it sealed with the U.S., European powers, Russia and China. These moves started over a year after the Trump administration exited the deal in May 2018 and then imposed broad sanctions on Iran, which had been lifted under the agreement.

It has also threatened to restrict IAEA inspectors’ access to sites starting later this month. These steps have caused growing concern in Washington about Iran’s nuclear intentions.

U.S. and Israeli officials have said Iran’s retention of nuclear material, equipment and information, contained in a nuclear archive raided by Israel in 2018, show the country plans to rev up its nuclear weapons work again.

The agency is also asking Iran about another undeclared site where illicit uranium conversion and processing may have taken place, it said.

All the suspected activities took place in the early 2000s or earlier, according to the agency. Two of the sites were razed years ago. Another site was sanitized by Iran in 2019, the IAEA reported. The IAEA said it has not ruled out that materials from this nuclear work have been used more recently.

“The discovery of radioactive material at these sites would indicate that Iran does indeed have undeclared nuclear material, despite its denials,” said David Albright, a former weapons inspector and president of the Institute for Science and International Security in Washington. “It would indicate that Iran did have a nuclear weapons program in the past, likely leading the IAEA to call for access to more sites and more explanations from Iran.”