Iran has begun paving over a former military site where its scientists are suspected to have conducted nuclear-weapons-related experiments, according to a new U.N. report, a move that could doom efforts to reconstruct a critical part of Iran’s nuclear history.

Satellite photos of the site, known as Parchin, show fresh asphalt covering a broad area where suspicious tests were carried out several years ago, the International Atomic Energy Agency said in an internal report that was prepared for diplomats.

The paving appears to have occurred within the past few weeks, at a time when the United Nations’ nuclear watchdog was meeting with Iranian officials to try to negotiate access to the site to investigate allegations of secret weapons research.

Iran has repeatedly denied IAEA inspectors entry to the site, and previous satellite photos have shown a series of efforts to alter it by razing buildings and even scraping away topsoil around what was once a chamber used for military explosives testing. U.N. officials believe that the facility may have been used to test a special kind of detonator used in nuclear explosions.

Since February, Iran “has conducted further spreading, leveling and compacting of material over most of the site, a significant proportion of which it has also asphalted,” the IAEA said in its report, a copy of which was obtained by The Washington Post.

The alterations to the site “have seriously undermined the Agency’s ability to undertake effective verification” of Iran’s claims that its nuclear program is entirely peaceful, the report said.

Iran denies that it ever conducted nuclear weapons research and says the IAEA has no mandate for investigating a military base with no ties to its nuclear program.

The IAEA, which conducts routine monitoring of Iran’s civilian nuclear facilities, met with Iranian officials earlier this month in the latest in a string of failed efforts to clear up concerns over suspicious experiments by Iranian scientists. U.S. intelligence officials believe Iran was testing components for nuclear weapons as recently as 2003, when the work was abruptly halted.

Since then, Iran has amassed a large stockpile of enriched uranium, a key ingredient in nuclear arms, but has not decided whether to take the risk of building and testing a bomb, U.S. officials say.

The IAEA report also documented Iran’s continued progress in increasing its supply of enriched uranium, including the addition of still more advanced centrifuges that produce nuclear fuel more efficiently than the outdated machines formerly used by Iran. At the same time, Iran has continued to convert some of its uranium stockpile into metal fuel plates, a step that would make it more difficult to use the material in a future weapons program.