The United States says it is seeing "indications that Iran" is cooperating with inspectors from the UN nuclear watchdog.
White House spokesman Josh Earnest made the comments on September 21, after the International Atomic Energy Agency (IAEA) said "significant progress" had been made in its probe into the possible military dimensions of Iran's nuclear program.
IAEA Director-General Yukiya Amano said Iran had handed over environmental samples from the controversial Parchin military site.
He said Iran's role in the probe of the site and its taking of dust samples at Parchin met IAEA standards.
The samples will be analyzed for traces of nuclear elements that could indicate if high-explosive experiments were conducted at Parchin.
Amano also cautioned that "much work" remained before the investigation could be completed by December's deadline.