The Obama administration urged Syria on Thursday to come clean about its past nuclear research as well as its chemical arsenal, accusing President Bashar al-Assad of blocking access to facilities linked to a Syrian nuclear reactor destroyed by Israel in 2007.

The top U.S. diplomat to the International Atomic Energy Agency (IAEA) said Syria’s two-year-old civil war was no excuse for its failure to answer questions about its alleged nuclear program, which Western intelligence officials believe was on a path toward making nuclear weapons.

“It remains essential that Syria fully cooperate,” Ambassador Joseph Macmanus told a meeting of the U.N. watchdog agency’s 35-nation governing board in Vienna.

Macmanus specifically pressed for access to three sites inside Syria that he said were suspected of having a “functional relationship” to the Deir al-Zour reactor destroyed by Israeli warplanes six years ago. One of the sites has been described by U.S. officials as a pilot plant for making the reactor’s uranium fuel.

The three facilities have long been a focal point of an IAEA investigation into the size and scope of Syria’s nuclear program, which is believed to been halted by the 2007 Israeli air raid dubbed Operation Orchard. The presumed cornerstone of the program was the plutonium reactor built with North Korean help on the banks of the Euphrates River in Syria’s eastern desert.

A report Thursday by independent nuclear researchers said ancillary facilities built to support the Syrian reactor could still contain uranium and other material of potential value to terrorist groups or black-market profiteers. The Deir al-Zour reactor was still under construction at the time of the 2007 attack, and it is unclear what became of the hundreds of uranium fuel rods that would have been required to operate the facility .

“The uranium could be anywhere within government controlled areas today, if it even remains in Syria,” warned the report by the Institute for Science and International Security, a Washington-based nonprofit group. “Determining its fate must be a priority.”

The report acknowledged that any uranium fuel remaining in Syria is not weapons-grade and could not be used in nuclear bombs without further processing. While Syria’s thousands of chemical weapons remain a higher priority, its nuclear assets “deserve significant attention,” the study said.

The ISIS report included satellite photographs of one of the suspected ancillary sites, a large compound in Marj as-Sultan, east of Damascus. The facility was first identified two years ago in a German newspaper, Süddeutsche Zeitung, which also obtained what was purported to be photographs of equipment inside the facility as well as a large stack of metallic rods that resembled uranium fuel.

A technical expert who reviewed the images for ISIS said they appear to depict reaction vessels, manifolds and other machines commonly used in processing uranium fuel. ISIS acknowledged that it could not independently verify where, when and by whom the photographs were taken.

The collapse of government control over parts of Syria has brought new urgency to long-standing questions about the security of Syria’s remaining nuclear assets. In February, Islamist rebel militias swept through the town of Deir al-Zour and posted videos of gun-toting fighters clambering over the site where the reactor had stood. The Marj as-Sultan region also has seen fighting between rebels and government forces in recent weeks.

“If the United Nations Security Council places Syria’s stocks of chemical weapons under international control, it should also address Syria’s undeclared nuclear assets,” the ISIS study said. “The goal should be to verifiably determine that the undeclared program and its assets are fully understood, rendered harmless, and dismantled.”