AUGUST 18: U.S. Senator Bob Menendez (D-NJ) gives a speech announcing he will not support President Obama's Iran nuclear deal at Seton Hall University on August 18, 2015 - Getty Images

VIENNA (AP) — An unusual secret agreement with a U.N. agency will allow Iran to use its own experts to inspect a site allegedly used to develop nuclear arms, according to a document seen by The Associated Press.

The revelation is sure to roil American and Israeli critics of the main Iran deal signed by the U.S., Iran and five world powers in July. Those critics have complained that the deal is built on trust of the Iranians, a claim the U.S. has denied.

The investigation of the Parchin nuclear site by the U.N. International Atomic Energy Agency is linked to a broader probe of allegations that Iran has worked on atomic weapons. That investigation is part of the overarching nuclear deal reached earlier this month between Iran and six world powers.

The Parchin deal is a separate, side agreement worked out between the IAEA and Iran. The United States and the five other world powers that signed the Iran nuclear deal were not party to this agreement but were briefed on it by the IAEA and endorsed it as part of the larger package.

Without divulging its contents, the Obama administration has described the document as nothing more than a routine technical arrangement between Iran and the U.N.'s International Atomic Energy Agency on the particulars of inspecting the site.