The U.N. nuclear watchdog, which is investigating whether Iran carried out work related to developing a nuclear bomb, said on Sunday its chief had visited a sensitive military site during a trip to the country.

Environmental samples have been taken at a sensitive military site in Iran, the head of the U.N. nuclear watchdog said on Monday, citing “significant progress” in its investigation of Tehran’s past activities. The International Atomic Energy Agency (IAEA) Director-General Yukiya Amano said he and the head of the agency’s Department of Safeguards, which carries out inspections, visited a building at the Parchin site on Sunday that the agency had previously only observed by satellite.

The IAEA is due to provide an assessment of “possible military dimensions” of Iran’s nuclear program by the end of the year. That report is a vital confidence-building aspect of Iran’s landmark deal with six major powers reached in July, under which restrictions will be placed on Tehran’s atomic energy activities in exchange for a lifting of sanctions.The IAEA has drawn criticism over a confidential arrangement with Iran governing how inspections are done at Parchin. Critics of the international powers’ deal with Iran have argued that the accord on inspections limits the IAEA’s ability to investigate and gives Iran too much influence in the collection of samples.