A U.S. envoy accused Iran on Thursday of "systematically demolishing" a facility at the Parchin military site that United Nations nuclear inspectors want to visit as part of their investigation into suspected weapons research.

"Iran has been taking measures that appear consistent with an effort to remove evidence of its past activities at Parchin," U.S. envoy Robert Wood told the 35-nation board of the International Atomic Energy Agency.

He said it was "troubling that Iran is blatantly hampering the [IAEA's] ability to carry out its mandate by systematically demolishing the facility that has been identified by the IAEA as meriting inspection at the Parchin site."

Remarks come two weeks after IAEA report expressed concern over "significant ground scraping and landscaping" conducted in the military base. The report said five buildings had been demolished and power lines, fences and roads removed, adding that these actions could obstruct any investigation at the complex, if access is granted.

A few days before the report was made public, a U.S. think tank published satellite imagery showing a pink material covering a building at the military site. The institute for Science and International security said that the use of the tarp might be an attempt to conceal possible clean-up activity at the location, amid growing international pressure on Iran to open the site for inspection.

The IAEA believes that explosives tests relevant to the development of nuclear weapons were conducted in a steel chamber in the building, possibly a decade ago, and has repeatedly asked it would be granted access to the site.