Iran has for the first time ever provided the United Nations with environmental samples from its highly controversial Parchin military complex, although no U.N. inspectors were present, the head of the U.N. nuclear agency said Monday, reported Reuters.

The Parchin site is suspected to have at one point been used to conduct atomic weapon research, and the International Atomic Energy Agency (IAEA) has been working to confirm those possible "military dimensions." Iran maintains its program is for peaceful purposes only, but until now, had repeatedly rejected IAEA requests to visit the site. The IAEA is expected to release an assessment of the sample material by the end of the year, according to BBC.

IAEA Director-General Yukiya Amano said he visited a building at the Parchin site on Sunday, but "the Iranian side played a part in the sample-taking process by swiping samples," reports NBC News. Such sampling is normally carried out by the IAEA's own experts, notes the Times of Israel.

Amano said the samples were authenticated under "established verification process" and noted "significant progress."

The IAEA reportedly monitored the Iranians' collection of samples through video and still cameras, according to the Times.

Armano also suggested the Iranians may have done contruction on the site since the U.N. inspectors last visited in 2005, which may make it difficult to verify Western intelligence suggesting that Iran carried out tests involving explosive triggers for nuclear weapons.

"Inside the building, we saw indications of recent renovation work," Amano said, according to Reuters. "There was no equipment in the building."

The IAEA's investigation into Iran's past nuclear activity is separate from inspections to other sites agreed to under the July nuclear deal between Iran and six world powers. However, IAEA's report is crucial to the implementation of the agreement, which would see temporary restrictions placed on Iran's nuclear program in exchange for the lifting of international sanctions.

The IAEA and Obama administration have been criticized over secret side deals between Iran and the IAEA governing how inspections are done at the Parchin facility.

By allowing Iranian technicians to take samples themselves while IAEA experts observe and oversee the process, critics say Iran is allowed too much influence in the process.

Some lawmakers say President Obama broke the law by failing to transmit the text of the side deals to Congress, further contending that the entire Joint Comprehensive Plan of Action is invalid until Congress is allowed to view and vote on the full text, as HNGN previously reported.