The outlet said intelligence had satellite images that showed heavy machinery working at the site in broad daylight.
In a statement to the UN, Iranian delegates said these allegations were "preposterous" and aimed at derailing Tehran’s nuclear accord with six world powers that would see it scale down its nuclear program in return for sanctions easing.
"We regret that the extensive vicious campaign at work, using tens of millions of dollars, to poison the positive environment at the global level, which followed the conclusion of the JCPOA [Joint Comprehensive Plan of Action]," the statement said.
According to Iran, bulldozers spotted outside the military site were repairing a road near the Mamloo Dam.
"The Parchin military complex covers a very vast area, which includes different military, industrial, administrative and residential sections. It is, thus, quite normal to have construction works underway at any time," it argued.
Monitoring suspected nuclear military sites is an element of Tehran’s deal with the International Atomic Energy Agency (IAEA), agreed in mid-July on the sidelines of nuclear talks between Iran and six world powers alongside the July 14 agreement.